(Also defrag your drive regularly and make sure it is not overfull. The game needs a fair amount of free space to actually RUN.
SimEnhancer by Rick Halle (Pay programs but you CAN get a new registration code generated here if you have the details of your original purchase).
Lot Manager by SimWardrobe
Skin Doctor by Dr Pixel - sadly the Blarney Stone has closed, but you can get this program from the Sims Resource now.
The Sims File Cop - really only useful for walls, floors and clothes - its main function is to check that textures are in 16-bit format.
Transmogrifier by Don Hopkins
Homecrafter by Maxis
IFF Edit from SimPrograms
Script Station from SimPrograms
Career Creator 3 or Job Factory
FAR Out SimWardrobe
The Sims Menu Editor
FaceLift Gold by Maxis
The Sims Art Studio by Maxis
Sims Eraser from Maxis - run this as many times as you have EPs plus two when uninstalling - IF you want to uninstall Sims 1 and EPs it removes the Registry entries that can prevent reinstalls.