Dolphin

From the Dolphin homepage:

Dolphin is a Gamecube and Wii emulator. Most games run perfectly or with minor bugs. Games are playable at HD quality, with 1080p and more. This is a remarkable feature the actual Gamecube and Wii consoles are not capable of.

Wiki Page (includes game compatibility info): http://wiki.dolphin-emu.org/index.php?title=Main_Page

Downloads Windows Mac OS X Linux Android
Download page (with latest builds)