I'll admit it; I'm a freeroll junkie. I definitely agree with the people who said Merge (Carbon, Players Only
, Big Bet, etc.) has some of the best freerolls, though their schedule has changed in the past month, and they're running less of them.
I'll also play pretty frequently on Full Tilt. I like the $150 freerolls. Never really tried to play in the qualifier freerolls. I also try to play the sats for Daily Dollar and Ferguson tourneys -- they're pretty worth it, though I can't say I've had much of a run in either tourney once I've qualified. (The fields are HUGE).
On Stars, I don't really play the freerolls unless it's an interesting game like Badugi or unless it's one of the Daily 90k Sats. As has been said, it does offer a ton of round 1 freerolls. The FPP freerolls aren't too bad, either.