AppleTV 1 and 2 offers compatibility with any video you can rent or own from the iTunes store.
They offer means of getting iTunes, or iPhoto (via iTunes) to either sync (on AppleTV 1) or stream (AppleTV 2) content to them.
They offer the means of streaming Netflix in stream queue (on AppleTV 1 and 2), and searching and selecting streamable Netflix content (AppleTV 2 only).
They do not offer any ability to show the actual operating system display from your Mac.
Both offer Youtube video support.
All G5 and Intel Macs have had built-in DTS sound support and at minimum DVI support.
AppleTV 1 supports Mac OS X 10.3.9 or later with iTunes 7.7.1.
iTunes 9.2.1 supports Mac OS X 10.4.11 or later.
AppleTV 2 supports Mac OS X 10.5.8 or later.
That said, a couple of third party wireless solutions support HDMI wireless up to 30 feet line of sight from your computer:
Macs which don't have audio over mini-Displayport connectivity, need either an
to get full surround audio into the HDMI port after connecting through the Mac's compatible audio port and adapting to the compatible video port.
For some DVI Macs, one of these mini-DVI to DVI adapters is needed to connect to a male to male DVI cable before connecting to the Apogee adapter
if they don't have the wider port shown below:
Note, I haven't tried any of these products except AppleTV 1, and AppleTV 2