DOOM Classic, id Software's iOS port of the seminal 90s first-person shooter, has just been updated to version 2.0. It's a fairly major update that brings support for Retina Display graphics and support for the iPad and iPad 2. At last, DOOM Classic is a universal app that you can play on the iPad.
I never played DOOM on the PC back in the day -- never had a machine that could handle it -- but I played through the iPhone version last year and enjoyed it a lot. I'm looking forward to giving it another runthrough on my iPad.
DOOM Classic is US$1.99 on the App Store. If you already own the game, the update to Retina Display graphics and iPad support is free.