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
5 days later
do they take forever to update? like OpenEmu?
OpenEmu's core emulators are ports of libretro. Libretro and Retroarch are ports of other emulators into one "library". Retroarch has had 5 0.0.x releases this year so far, and nightlies are available as well. Its my current emulator of choice (but on a Raspberry Pi 2 for now
I'll give it a go...edit: I'm so lost. lol Back to Openemu...until there's a video on Youtube that takes Mac users through Retroarch. Windows only on YouTube so far.
5 days later
Whats got you hung up.

Download the DMG file. Cpy to applications folder.

Launch

Add cores you want

Add your roms

Load Core

Go to the right to the controller icon and play your games
The interface is identical across platforms.
Just to verify, you did download retroarch, and not just the libretro libraries, right?
4 days later
Interface could be better with a mouse clicking on everything. I use a keyboard only for my video gaming, I find it easier.
Funny, I prefer it this way as I can use just my controller.