Nicky Boom 2 is a classic platformer with some puzzle elements thrown in. The game was ported from the PC in the DOS era and it's prequel is also available on the App Store (but wasn't published by DotEmu).
Golvellius - The Valley of Doom is a title reminiscent of the earlier 2D Zelda series. It is an action RPG with puzzle solving that was ported from the Japanese MSX and Sega Master System.
Earth Defense force is a side-scrolling shoot-em-up in space reminiscent of Konami's Gradius titles. The game traces its roots to the 90s arcades and the Super Nintendo.
Avenging Spirit is a 2D platformer with a twist -- you can take the form of your enemies. It's origin lie in the japanese arcade.
64th Street - A Detective Story is another arcade port that offers beat-em-up style of gameplay. The game is similar to the Double Dragon games from the 90s.As you can see, there are quite a lot of titles here offered for retro fans and gamers who grew up or who miss the 90s gaming scene. Most of the titles here also fit the pick-up-and-play approach of iOS' mobile devices -- especially the titles originating in the arcades. Let us know which games are worth keeping on that flash memory drive in the comments.