AirParrot is a straightforward tool that fills in a big hole in AirPlay's functionality: it can mirror your Mac's display on any AirPlay-compatible device. That means your AppleTV or iDevice, or even another Mac if you're utilising another tool called AirServer.
We've mentioned AirParrot before in our look at how to get the features of OS X Mountain Lion right now but never highlighted explicitly. It's very much worth a look if you want to be able to mirror your Mac's display (and audio) since it handles the job very well.
It can even let you extend your desktop to the other device if you need a little more space and optimise your video quality so you can find the right data rate for your network.
AirParrot was designed to work with OS X 10.7 Lion, but is also compatible with Snow Leopard. It's $US10, but there's a free trial if you want to give it a shot before you buy.