Bring up the cheat box by holding down the CTRL, SHIFT, and C keys on your keyboard at the same time. If you don’t get the cheat box when you do CTRL, SHIFT, C, check out Game Help:Cheat Box Won’t Appear.

apartmentBuildBuyRestrictions [on/off] (Apartments ep)
Enables the locked build and buy tools in apartment lots.

boolProp allow45DegreeAngleOfRotation [true/false] (Uni, Nightlife, Pets, OFB)
Allows users to rotate any object in 45 degree increments. Use the < and > (, and .) keys to rotate the objects; clicking and dragging to rotate will only rotate in 90 degree increments.
* Sims may have problems using some objects placed at a 45 degree angle, like televisions and appliances. Plan for objects placed at an angle to be purely decorative.

boolProp snapObjectsToGrid [true/false] (Base Game)
Toggles the ability to fine-tune the placement of items, instead of being limited to just placing them at certain spots on the grid.
* If you find you can’t place windows and doors because they won’t snap properly onto walls, disable this cheat (and/or remove it from your user startup.cheat file) and try again.

deleteAllFences (Pets)
Remove all fences from the lot. You must be in build mode to use this cheat.

deleteAllWalls (Pets)
Remove all walls from the lot. You must be in build mode to use this cheat.

deleteAllHalfWalls (Pets)
Remove all half-walls from the lot. You must be in build mode to use this cheat.

deleteAllAwnings (Pets)
Remove all awnings from the lot. You must be in build mode to use this cheat.

deleteAllObjects [Doors/Windows/Stairs] (Pets)
Removes all build mode objects of the chosen type from the lot. You must be in build mode to use this cheat.

individualRoofSlopeAngle [15-75] (Pets)
Similar to the roofSlopeAngle cheat from Nightlife, this cheat allows you to change individual pieces of roof, separate from the other roof parts. To use, first build the roof. Then enter the cheat. Acceptable values are 15-75 (degrees). While the roof tool is still selected, hold down CTRL and ALT and click on the the section of roof that you want to change. The roof will change angles.
* This cheat was broken when Pets shipped – you could change the angle but it would not save with the lot. You must download and install the Pets Patch for it to save properly.

Multi-placement on walls –> not entered into the cheat box!!! (Pets)
Press the CTRL + ALT keys while trying to place an object on a wall. Placement rules on walls will be removed, and you will be able to place any wall object on a wall already occupied by another object.

roofSlopeAngle [15-75] (NL, OFB, Pets)
Adjust angle of all roofs on the lot. Acceptable values are 15-75, which seem to equate to the angle in degrees.

changeLotZoning [residential/community/greek/dorm/secretsociety/apartmentbase/secretwitchlot] (NL, Uni, Pets, OFB, Apt)
Changes the zoning type of the lot. Before rezoning a lot using this cheat, make sure there are no Sims currently living in the lot. To use this cheat, enter the lot that you would like to rezone, and then enter the cheat, choosing one of these zoning options. Save and exit to neighborhood and re-enter the lot.
How do I change a lot to dorm, greek house, secret society, residential or community type?
This Build Mode cheat is enabled in University. You can not use this if you just have the Original Sims 2 installed.
To use the cheat, bring up the cheat box by pressing CTRL-SHIFT-C then type:
* changeLotZoning dorm
* changeLotZoning greek
* changeLotZoning secret society
* changeLotZoning community
* changeLotZoning residential
That will change the zoning of the lot. You will need to save and exit to the neighborhood for it to actually change.
* If you change an existing lot to a residential, you will need to give it a mailbox and a trash can for the lot to work properly You can find those items in a buyable collection here (click on Sims 2, then Hacks/Mods).
* If you create a new secret society lot, they will not be visible while in Neighborhood view and you will not be able to edit it again after changing the lot type. So be careful with that one!

boolProp dormSpecificToolsDisabled [true/false] (Uni, NL, OFB, Pets)
Enables build/buy mode tools that would normally be disabled when in a dorm lot.
*This will also allow you to enable build/buy mode when on a community lot, though your changes will not save with the lot. This function works even if you do not have University.

setHighestAllowedLevel [number] (Uni, NL, OFB, Pets)
Sets the amount of floors allowed (so you can build higher). You cannot go higher than 16 or the game will crash.
* The Dormer Trick: If you do not have any expansions (you only own the base game) OR if this cheat does not function for you (it appears to work only occasionally or not at all in later expansions) then you can try the following trick:
o Build up to the highest level you can.
o On that top level, place a dormer (they’re in with the roofs and such). This will let you build one more level up.
o Put a wall or something up on that next level.
o Remove the dormer (ctrl-click and drag with the wall tool, remove roofs).
o Continue building on your new level.

boolProp lotTerrainPaints [true/false] (Base Game)
Set to false to turn off viewing terrain paints (ground cover) on the lot. This cheat does not appear to remove them, just makes them invisible.

boolProp lotWater [true/false] (Base Game)
Set to false to remove water (ponds) on the lot.

boolProp lockTiles [true/false] (Base Game)
Set to false to place floor tiles at the very edge of the lot and to change the tiles of the road and sidewalks.

moveObjects [on/off] (Base Game)
Allows you to move and delete objects in build mode that you cannot normally move or delete. Mainly used in building to overlap objects to create interesting effects.
* Use with caution: deleting certain items such as the mailbox, trashcan, and community phone can cause problems with a lot.
* Deleting a sim living on your lot, saving, and exiting will result in that sim being restored and standing at the mailbox when you re-enter the lot, which can be useful if they’re stuck or otherwise bugged.

boolProp constrainFloorElevation [true/false] (Base Game)
Toggles the ability to change the elevation of floor tiles in areas that already have walls, floors, and foundations on them.

Source via

Custom Painting
To decorate a Sim’s house with custom artwork, your Sim must first have a creativity level of 5 and be able to paint a still life.
Have your Sim paint and a frame will appear that you can center on whatever you want to paint. If you want to paint a picture of something outside the Sims 2 game, just press C anywhere at this time. This will pause the game.

Use Alt + Tab to exit the game, then open a picture program such as Paint and open the snapshot you just took, it will usually be in the folder: C:\My Documents\EA\Sims2\Storytelling\Snapshot.bmp

Then, you also have to open the picture you want to have in your game. If your picture is too large resize it so it will fit in the original snapshot from the game.

Copy your custom picture on to your snapshot from the game, then save the picture. Use Alt + Tab to get back to your game and your sim will start painting your own custom picture.

Using this trick you can put pictures of your friends or family, web pictures, or anything else in your Sims 2 game.

Listen To Mp3′s On Sim Radio
First get whatever Mp3′s you want to put on the game, then open up MyDocuments/EAGames/TheSims2/Music/(whatever RadioStation you want your song on). Then, Drag your MP3 into the folder. Now, start Sims 2 then go to options menu, then music settings, then the Radio Station, then pick witch songs you want the radio to play. You can now listen to your favorite songs in your Sims game.

Use your own videos for the TV
It’s possible to watch your own videos on the Sim-TV. You must put the video you want into My Documents\EA Games\The Sims 2\Movies\Broadcast (the TV sender you want). If you then turn on the TV in the game you can watch it! The video must be .avi-clips and the clips shouldn’t be larger then 128×128 pixel also it shouldn’t have more then 15 frames per second.

Listen to music included with game
First, open your The Sims 2/TSData/Res/Sound directory. The names of radio stations, plus a few more including Build mode and Neighborhood music should be here in *.package files. Copy and paste the file corresponding to whatever you want to listen to into another folder. Rename it from [File].package to [File].mp3. You can now listen to the sound/songs included in the file.

Money Trick for Poor Families
The trick is that no matter how many times your Sims use an object, if they sell it in the same day, your Sims can get all their Simoleons back. So if your Sim does not have much Simoleons and needs body points for instance, get the Sim to buy a body machine and keep selling it back to refund all their Simoleons.

Saving Pizza
Have a Sim call for pizza. When the pizza delivery guy comes DON’T accept the pizza! The pizza delivery guy will eventually leave the pizza on the front porch, and the pizza will never get stale. You can stack as many pizzas as you want, so when your Sims get hungry, just instruct them to get a pizza from the front porch.

Any other Sims 2 tips & tricks?

Bring up the cheat box by holding down the CTRL, SHIFT, and C keys on your keyboard at the same time.

If you don’t get the cheat box when you do CTRL, SHIFT, C, check out Game Help:Cheat Box Won’t Appear.

maxMotives (NL, OFB, Pets)
Maxes the motive bars of all sims currently on the lot (controllable and uncontrollable). Not useful for business owners, especially restaurant owners (who wants customers that aren’t hungry?).

kaching (Base Game)
Add 1000 simoleons to the current family’s funds.

motherlode (Base Game)
Add 50,000 simoleons to the current family’s funds.

familyFunds [familyname] [amount] (NI, OFB, Pets)
Sets the chosen family’s funds to the given level (does not add the amount given, but sets it). If the family name has a space in it, you can put it in quotes – i.e. to set the di Marco family’s funds to 30000 you’d do: familyFunds “di Marco” 300000

forceTwins (OFB, Pets)
When a Sim is pregnant, this Sim has to be selected and the cheat entered. The Sim will automatically deliver twins when they give birth.

unlockCareerRewards (NL, OFB, Pets)
For the currently selected sim, they immediately gain access to all career reward objects in the catalog.

aging [on/off] (Base Game)
Disables aging for all sims on the lot, including children and babies. Also pauses any current pregnancies.

agesimscheat [on/off] (NL, OFB, Pets)
Enables “Set Age” when clicking on a sim, which will let you easily change their age group (and allow you to de-age them as well) The “Set Age” interaction will also appear on all objects until you turn off the cheat.

boolProp testingCheatsEnabled [true/false]
(Base Game)
Warning! This cheat is for advanced users, and improper use of it may cause problems in the game. Use at your own risk!!!
Used by Maxis for testing certain aspects of the game. This cheat has many, MANY uses, not all of them useful for the player, and some of them potentially harmful. If you must use the testingCheatsEnabled cheat, only enable it for exactly as long as you need to use it, then turn it back off. You will likely experience errors if you leave this cheat on – the cheat itself does not cause errors, but shows you errors that were happening anyway. If you do get errors, click CANCEL or RESET on them.