MAME OS X is the Mac OS X port of the multi-arcade machine emulator (MAME). It is an emulator that can play lots and lots of old arcade game ROMs. There are some current games that can also be emulated. Although the port for Mac OS X isn't as regularly updated as the Windows counterpart, there is still some fun to be had with it. The last update was in late 2009.
When you first run MAME OS X, you have a list of all the supported games (ROMs). The version I tried was current to the 0.135 revision. And all the ROMs that I had were supported. If you want the application to scan your ROMs directory and filter the list down to the games that you have, you have to carry out an audit. This takes while, but it is something that saves you time. You can play games while you are doing this, too. And I was surprised at how fast the games ran, while auditing.
In terms of performance, I really was surprised at how well the emulation was done and how smooth the overall experience was with MAME OS X, I tested it with several ROMs and I used some processor-intensive ones.
There is support for multiple keyboards and joysticks and there are good configuration options.
Although this might not be regularly-updated, it still works great and is a great app to have on your Mac. The only feature that I wish it had is support for saves.