Honestly it's your best bet to download a ton of programs that work in your area (If you're outside of the US: Pokerstars, 888Poker,
partypoker, Fulltilt etc...) then to scour all of them and this forum for freerolls.
Quickly:
Cardschat: Easily the best one. Regional and locked off freerolls are a great opportunity to make money as you can sometimes get $100-$300 in a freeroll that's split between 140-500 people which almost prints money after enough time if you're confident in your MTT skills.
888Poker: 2nd best. A large variety of freerolls from satellites granting entry to $5.50 and $33.00 buy-in satellites to $20 SNAP (ZOOM on Pokerstars). The big negative here is that most of them have a total entry volume of 800-2500 people, making it a lot harder to actually see the money/ticket.
Pokerstars: 3rd best. A few $10 freerolls from an invitation to their Pokerstars school project. Multiple freerolls to several hundred dollar entry tournaments, except there is only 1 ticket in the prize pool and roughly 1200-3000 entries.
Partypoker: 4th best. I found freerolls here to be almost non-existent; they also require a method of deposit to enter them, which dissuaded me. Still worth it if you can find some password protected/region locked freerolls happening here.
Full Tilt: Unknown. I've never used Full Tilt, if anyone has any information here I would appreciate it.