I wanted to quickly announce that I've released v0.13 of my NES emulator and debugger for Mac OS X 10.5.6 and later, Macifom. This version features highly accurate sound care of Blargg's excellent Nes_snd_emu library. Additionally, I've made some tweaks to the scanline renderer to eliminate minor scrolling glitches in Zelda and Super Mario Bros.
Macifom is written primarily in Objective-C to take advantage of robust, modern OS X frameworks such as AppKit and CoreAudio. Compared to Nestopia, it's rather immature. Currently only NROM, UNROM, CNROM, AxROM and MMC1 mappers are supported and all player input is via keyboard. Many more fixes and improvements are on the way, however, and I'm hoping to accelerate releases by building out a team of developers. If you're interested in contributing, please let me know.