For... *cough*... legitimate DivX backups of my properly purchased, fully legal, non-sueable DVDs, I use Kazaa Lite K++, the last free version before it went semi-corporate/got fucked etc. And that's safe, in that I've never caught anything shit from a .avi before.
Unless there's some file-merging fuck out there now laughing at my expense.
Anyway, besides that, er... some sites of questionable legality that shouldn't be mentioned. And otherwise I use the sites of the authors of the stuff I'm downloading. I find download.com etc slow, with a limited choice.