Poker Stars, by far the biggest and most popular online poker room, is not available as a free bankroll in very many places. If it was it would probably prove to be the most popular free bankroll promotion ever.
Any post on the site which includes a reference to Poker Stars will be listed here.
PokerStrategy
PokerStrategy is one of the biggest online poker communities and offers a lot more than just free bankrolls. The way PokerStrategy handles their free cash promotions is different than the other bankroll providers that you may be used to.