Absolute Poker has the most freerolls... but majority of them are pretty small.
If you register you will be entered into a $1000 freeroll (no deposit needed). Since only newly registered players are allowed in (you can only play a new registration freeroll once) this freeroll is very lucrative... $1000 prize pool and only a couple hundred entrants if I remember correctly.
Once you're registered at Absolute they've got 40 freerolls or something crazy every day. at
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You can play like 5 of these freerolls all at once and win good money
The main
http://www.freerollz.com is definetly the best reference for freerolls