Touch Foo recently released a free Lite version of their iPhone platformer Soosiz. We loved the game in our review, declared it the best iPhone platformer and gave it 5/5 stars in our monthly roundup of the best October iPhone games.
The big twist to the game is its gravity-changing mechanic. When jumping to a platform above you, the entire world spins around and up becomes down. While this all seems like it might just be a novel gimmick, Touch Foo manages to utilize it fully across Soosiz's 7 worlds and 65 levels. The gravity experience is truly an integral part of the game, and not simply a tacked on afterthought. Triggering this gravity manipulation simply requires you to jump onto another platform, and becomes second nature very quickly.
The full version contains 65 levels with progressively more difficult and interesting levels. The Lite version only gives you the first 5 levels, so while you'll get a good taste of the controls, the levels remain very straightforward. Still, the Lite is a "must try" if you managed to miss the game before.