A few words of advice:
1) Building a bankroll from nothing takes forever. If you don't have serious patience to play for ~5-6 months earning pennies at a time, you won't make it.
2) Playing tons of freerolls
will harm your game. It will be somewhat difficult to make the adjustment to playing for actual money.
3) You should pick a site not just on the amount of freeroll money they give away, but also their microstakes options. For example, Full Tilt runs a lot of freerolls, but their options for playing with that money after you make it are horrible. The lowest buy-in sit & go at full tilt is 1$, and it has a 25% rake. At Ultimate Bet, for example, they have $0.50 sit & go's that have a 5 cent rake (10%). So its much easier to build your bankroll after you win a couple of freerolls.
4) In my experience freeroll whoring, I'd probably start with pokerstars
, bodog, ultimate bet, or carbon poker
5) Play private freerolls. Cardschat & other sites offer freerolls that have vastly reduced fields so its much easier to cash.
If it were up to me, what I would do is this: Play in freerolls until you cash in one at whatever site you like best. Then, use those freeroll winnings to supplement your first deposit. Playing freerolls will build up some discipline, however, its just terribly slow at building bankroll.