For all you Mac users like me who can't use AirPlay Mirroring. This is a program that will do that on Macs or PCs running OS X 10.6.8+ or Windows XP and greater and AppleTV 2 or 3. There is a free trial you can use to test it out as well.
I use it with a Mac and you can mirror your desktop, use your tv as a second desktop or put just one app on the screen. Sound works with this as well.