Silverfish is an arcade shooter with an interesting twist: you don't actually shoot at all. You control a little bug-like ship (kind of gross, actually, if you've ever been plagued by the real thing) that you can move around the screen by swiping up, down, left, or right. You're chased by enemies just like more traditional shooters, such as Geometry Wars, but instead of shooting them yourself, you need to hunt down little proton pellets. Touching those explodes them and kills enemies, so you've got to time it just right -- trigger an explosion too far away from the bad guys, and you won't hit them, or if you get too close, you'll get hurt. Of course, various enemies show up over time, and the game is an interesting experiment in a new arcade variant.
It's pretty tough, actually. I had to play around with it for a bit before I really started picking it up, and even then I didn't come anywhere near the Game Center leaderboards. There are also achievements, and three modes as well. Reaper mode lets you build up a POW meter to destroy enemies with your ship, Onslaught mode pits you against waves of enemies on your own, and Scavenger is a sort of survival mode that tasks you with collecting protons in order to keep your life up. They all play pretty much the same (move away from enemies and towards protons), but the twists are good enough to put a new spin on gameplay.
Silverfish is a great twist on arcade action, and it's a steal at US$1.99 if you're into non-traditional shooters. That's an introductory price, so if you're going to pick it up, do so soon.