I know more than a few people who connect an older Mac to their TV, using various audio and video cables, to play videos. Most of these people would love to instead stream those videos wirelessly to an Apple TV, but their Macs aren't new enough to support Apple's AirPlay technology and the AirPlay mirroring feature of Mountain Lion (OS X 10.8). Sometimes they stream video using iTunes, but they also have video files in formats iTunes doesn't support.
Unless Apple works some unexpected magic with OS X, older Macs will never get official AirPlay-mirroring capabilities. And I don't expect iTunes to start supporting more video formats any time soon. But thanks to the $15 Beamer, AirPlay streaming is still possible. This simple app lets you stream videos from older (and newer) Macs to a 2nd- or 3rd-generation Apple TV. Specifically, Beamer works with 64-bit Intel Macs (any model from 2007 or later, along with some 2006 Macs) running OS X 10.6 or later.
Launch Beamer, and its window shows all compatible Apple TVs on your local network. Choose one, and Beamer instructs you to drop a movie file into the Beamer window; a few seconds later, the movie starts playing on the chosen Apple TV. (One feature I'd like to see is queued playback, so I could drop a group of videos onto Beamer and have them play back in order.)
Beamer streams the video to your Apple TV just as if you'd streamed it from iTunes: full-screen, with a progress bar whenever you pause playback or use the forward or rewind controls. In fact, you can use your Apple TV's remote to control playback--you don't need to do so from your Mac. If your video file includes subtitles (or is paired with a subtitles file) in MicroDVD, SSA/ASS, SubRip (SRT), or SubViewer formats, Beamer can display those subtitles during playback, although you can't adjust the size or font.
But Beamer isn't just for older Macs: It's also useful for newer Macs running Mountain Lion. Even though these newer models support AirPlay mirroring, Mountain Lion mirrors the entire OS X interface--watching a video over AirPlay mirroring isn't as elegant as using Beamer, which streams just the video itself (and, as mentioned above, lets you use the Apple TV's own remote to control playback). Beamer also supports more video formats than streaming from iTunes: AVI, FLV, M4V, MKV, MOV, MP4, and WMV (though not DRM-protected videos). You can even export Keynote presentations as movie files and present wirelessly. And if your AirPlay-mirroring Mac is just barely AirPlay capable, you may find that streaming a video using Beamer looks better and suffers from fewer performance issues.
Unfortunately, Beamer can't stream DVDs (although it can stream the individual VOB files contained in a ripped DVD's VIDEO_TS folder). And Beamer converts multi-channel audio tracks to stereo, so you lose surround-sound effects. (The developer's FAQ says future versions of Beamer will support multi-channel audio.) Finally, the developer says that sometimes Beamer may have trouble streaming extremely large files, although I experienced no issues with ~1.5GB .m4v files streamed from a 2010 Mac mini. (I've seen a few reports of problems with files 4GB or larger.)
Beamer does only one thing, but in my testing does it well. And at $15, it's a lot cheaper than buying a new Mac just to get AirPlay capability.