Hey guys,
I've recently been playing around with http://www.libretro.com/ on iOS and OS X -- looks like a nifty alternative to OpenEmu.

Downside: it doesn't tie in to many Mac/iOS specific APIs
Upside: it doesn't tie in to many Mac/iOS specific APIs.

Basically, the thing has a keyboard driven interface, but it's an active project, and includes support for many different emulators (and even has a Minecraft clone core). You can do rewinds and step-throughs, and the save states can be done on one platform and picked up on a different system.

As far as I know, it's currently the only emulator that will run PSX games on a non-jailbroken iOS device. There is also ongoing work to provide modern OpenGL and shaders support across all emulation cores, and a continual flow of ideas between emulators (providing some systems like Mednafen with features the original didn't have, because it inherits it from other cores in LibRetro).

Setup can be a bit tricky; you need to get all the right roms and databases loaded into the systems folder, and only certain builds work on certain systems -- but I successfully have it running in windowed mode on my MBP, and I've got it working on my iPhone Plus as well, by downloading the core libraries to my Mac and signing them with my own cert, then deploying them to the cores folder, and then selecting one of the iPad onscreen overlays for the buttons (the default doesn't work at all).

for a list of all the emulators supported and a description for them, see:
https://github.com/libretro/libretro-super/tree/master/dist/info