Mountain Lion 101: AirPlay Mirroring
AirPlay Mirroring is a great way to share content from your desktop, no matter what program you're running. It works well even for streaming video sites like Vimeo.
You can control AirPlay Mirroring via the new AirPlay menu bar icon, which shows up when there's a valid target device on the network. You get some additional display options once you've turned AirPlay Mirroring on:
Other than this issue, I think that AirPlay Mirroring is going to be extremely useful. The requirements are pretty tight; if you have a computer that wasn't purchased in the past year or so, it probably is not supported. If you're looking to get AirPlay Mirroring features for older systems, AirParrot will fill that gap nicely.