Sims 2 has the largest user made content community, it's almost endless.
For a while now I've been using Sims 2 models in my Machinima and I finally got round to writing a tutorial so others can do the same.
You can find the tutorial on my blog.

Hope it's useful.
If there's a need for it I might get round to compiling a list of good Sims 2 sites later.