What applications enable AirPlay mirroring for Mac? Do any applications enable AirPlay from my Mac to an Apple TV? I am looking to wirelessly mirror my Mac's display to the TV, similar to the way iPad 2 and iPhone 4S can mirror their displays over AirPlay.

Yes - it's coming as a feature for Mountain Lion, but solutions that work now on Lion are much appreciated.
With the new OSX Mountain Lion you will be able to airplay from the Mac to an Apple TV.

http://www.apple.com/macosx/mountain-lion/features.html#airplay