The Sims/The Sims 2:
Maxis: The Sims: The official The Sims website for all editions and expansion packs.
Aspyr: The company which ported The Sims, expansion packs and The Sims 2
The Sims Resource: One of the best places to find skins, objects, etc.
The Well Dressed Sim: Another great site for user created content for Sims and Sims 2.