From SnootysimsBelow links to several Sims 3 programs and guides. Also sites where you can download mods and sites where mods are discussed and linked to.
A note on mods: All mods can be used safely; in the worst case, your game may be unable to load. If that happens, simply remove the mod. Also note that some mods are adult in nature (like the InTeen mod). So use those on your own risk.
If the mod requires you to overwrite a file, ALWAYS backup the original!
Having given these warnings, I do not feel responsible for any problems you may have with the game. Nor for being blamed by angry parents for raising the games' teen rating to adult.
http://www.snootysims.com/index.php?id=programs