AirParrot lets you mirror your desktop, specific apps, and even lets you extend your screen on to a big screen via Apple TV, but it isn't perfect.
Particularly compelling just after the release of Mountain Lion, AirParrot lets Mac users with machines running Mac OS X 10.6 Snow Leopard or later mirror their desktops (Mountain Lion's AirPlay Mirroring only works on Macs made after 2011). The app creates a Menubar item where you'll be able to mirror your desktop on a big screen via a second- or third-generation Apple TV. Once you're on the same network as your Apple TV, simply use the drop-down menu to choose it and you'll begin mirroring what is on your desktop currently.
The app comes with several settings to play around with in the program's settings, including the ability to change resolution, video quality, and frame rate, along with controls for underscan to make content fit your TV screen.
AirParrot has a couple of extras that will be useful as well (that are not included in Mountain Lion's AirPlay Mirroring). You have the option under the Menubar drop-down menu to choose Extend Desktop, making your TV into a second desktop where you can drag-and-drop files, windows, and whatever you want on to your TV screen. There's also an option to run a specific app on the extra screen, but in my testing it made performing other tasks on my computer a bit laggy and jittery while AirParrot was running.
So, how does it compare with Mountain Lion's AirPlay Mirroring? It's not nearly as smooth. Loading up a Hulu Flash video was easy enough, but the video was fairly jerky and not as high resolution as using Airplay Mirroring in Mountain Lion. Even so, if you're on an older Mac that doesn't work with AirPlay Mirroring, AirParrot is pretty good alternative.