Apple - Is there a way to stream video to an Apple TV without iTunes?

Indeed there is. You can try AirFlick - it encodes video on the fly and sends it straight to your AppleTV. I've used it for a while now, and although it can be a little unstable from time to time, it really is very useful.

Of course, you could always install XBMC following this guide and control it, play all sorts of video, and make tons on neat customizations however you see fit.

Good luck.

Airvideo is another option, but requires another iOS device to act as an intermediary between the atv and the machine where your video is stored.

Firecore is brilliant, it allows almost any sort of video to be streamed from a networked drive directly to the ATV without the need for ANY intermediary device (I have only tried it with a Time Capsule). BUT it requires a jailbroken ATV and is currently in Beta. Easy install with a very simple bundled jailbreak package. Still, does away with the need for iTunes or an 'always-on' computer.