This seems like the kind of thing that Apple wouldn't want on the App Store (and indeed, it may soon disappear when they figure it out), but then again, after the relaxing of the rules, maybe they're OK with this one. Touch Arcade has discovered iDOS on the App Store -- it's a DOSbox-based disk operating system emulator that runs on iOS. And it apparently does everything that DOS does, including, as you can see in the picture, running Windows 3.0 complete with the Solitaire game.
The app is a 99 cent universal app for iPhone and iPad, and unfortunately, it's not user friendly at all -- if you don't remember your old DOS commands, the app's not set up to make things easy for you. The app also includes what the description calls "freeware/abandonware" like Ms. Pac Man PC, and DigDug, but you can upload other games and files through iTunes.
Honestly, this is awesome, but it seems a little too open for Apple's tastes. I can't see how it would ever hurt the iOS device (you're running everything from within the app, and Apple could simply assume that you own any copies of software that you install on there), but this is one app that may have snuck through the gates. At any rate, iDOS is now available on the App Store -- if it disappears later on, we'll let you know.