I downloaded the VisualBoyAdvance Gameboy Advance emulator last night and was pleasantly surprised to see near-perfect emulation, even with brand new GBA games like Sonic Advance 2 and Tony Hawk 4. (You can find ROMs in alt.binaries.emulators.gameboy.advance or on the web, if you look hard enough.)
Likewise, I tested out the Nemu Nintendo 64 emulator with Mario 64 and Zelda: Ocarina of Time. Zelda was playable despite display bugs, and Mario looked nearly perfect. (Try alt.binaries.emulators.nintendo-64 for ROMs.) Update: Apparently, Project64 is the most popular and compatible N64 emulator to date, but I haven’t tested it yet. These compatibility lists are very useful.
The NSX2 Playstation 2 emulation project managed to get the loading screens from Mortal Kombat 5 and Blade 2 to display. As far as I know, it’s the first PS2 emulator to partially load a commercial game.
Finally, gCubix is an amazing Gamecube emulator that’s been ported to a number of platforms, including Windows, Mac, BeOS, AmigaOS, Apple II GS, Commodore 64, Atari ST, PDP-1, and Babbage’s analytical engine.