Boxee (From iOS Device):
Up until recently, I loved the Boxee app on the iPad. It supports AirPlay, so using the iPad as a middle-device between a mac/pc with the video file and the Apple TV works great. However, it seems to drop the connection after 10-15 minutes from the computer, and the only way to get it back is to quit the Boxee Media Manager (server program) process and restart it… so no longer a good option.
Airplayit (From iOS Device):
Last night I had great success with Airplayit (Free). It isn’t as fast as Boxee, and definitely not as pretty, but seems solid as a rock, with no drop-outs or freezing during playback. Same setup as Boxee, install server application on Mac/PC, install app on iPad (Free, I have the iPad 2), connect, Airplay, enjoy virtually entire video collection on ATV3.
Plex (From iOS Device):
I haven’t tried Plex since there is no free trial (Same with a few of the other pay for ones).
AirParrot (From Mac/PC):
I purchased AirParrot for the mac/pc (Allows Airplay from a Mac or a PC), however I wasn’t able to get it to play airplay video to the ATV3 last night without it lagging out on my MacBook Pro (Lion, Core i5, 8GB of RAM). Will try again because non-video works fantastic. When Mountain Lion is released in a few weeks, I will try that as well.