Chillingo's Feed Me Oil [$0.99 / $1.99 (HD)] hit the App Store around a year ago, and our review shows, we had a great time with it. In essence, the game is a physics puzzler only this time around, the physics gimmick you're forced to harness hinges on fluid dynamics.
Every level features two things: A spigot where oil shoots from, and an area (usually designed as a mouth of a strange creature) where you've got to get the oil to. Completing each level involves getting the oil from the spigot to the goal area utilizing widgets you can drag into the game area. For instance, oil flows off simple platforms, it can be curved by magnets, and even blown in a different direction by fans.
Feed Me Oil was shockingly popular when it first hit last summer, so there's a decent chance you already have it. If this is the first you've heard of it though, make sure you don't miss this freebie.