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.