Mac: Streaming anything to your Apple TV is possible if you've got OS X 10.8 and screen mirroring enable, but it's not a perfect solution if you want to watch videos at the best possible quality (or haven't upgraded to Mountain Lion). Beamer solves that problem by streaming any file, converting it on-the-fly if necessary, to your Apple TV via drag and drop.
Beamer supports plenty of formats: AVI, MOV, MKV, MP4, WMV and FLV. You drag the video onto its minimal interface and wait for the streaming to begin. Beamer will even automatically detect your Apple TV and connect to it so you don't have to set anything up. As long as both devices are on the same network it will get the job done.
The main downside with Beamer is that it costs $US15. It's a little hefty for a single-purpose utility, but if you really need a simple streaming solution it might be worth the money. You can, of course, test it out first. The free trial allows you to stream 15 minutes of any video. If you're just streaming short clips, you can use it for free indefinitely.