Dolphin is a dual-purpose Wii and GameCube emulator, able to run homebrew code and commercial backups. This is the 64-bit flavor. Release notes from the developers involved can be found below.
As Dolphin nears the first release since becoming open source software, we are hard at work to make it more stable. Now, we offer up "v2.0 RC1" for our faithful testers to try out and report back!