Somebody should really write up a guide for this and just pin it at the top of every forum for a while--I've completely lost count of how many times this question has been asked recently.
A) VLC is free and will play ALMOST anything. The only thing it doesn't like are a handful of newer WMV files.
B) If you DON'T have an Intel Mac, install the free Flip4Mac codec that you can download from Microsoft's site. It plays anything Microsoft's old Windows Media Player for Mac would, and plays it MUCH better from within Quicktime. Windows Media Player is functionally useless now--it was such a piece of junk that Microsoft itself discontinued it completely and just paid Flip4Mac to give away their codec instead.
C) Continuing, if you DON'T have an Intel Mac, if you download and install the latest DivX codec and the latest 3ivx codec, you can now play 97% of all AVI files you find on the Internet. The majority of the remaining 2% can be played by VLC. The rest probably use the VP6 codec and will not play on a Mac, period.
D) If you DO have an Intel Mac, there isn't a version of Flip4Mac out for it QUITE YET. It's in beta, and will no doubt be out soon. If you're impatient, try downloading Popwire's free WMV9 codec in the mean time--with it installed, Quicktime will play many WMV files, and VLC will play most of the rest.
E) Continuing, also install the DivX codec (which is Intel compatible) but NOT the 3ivx codec, which is not. If desired, I believe there are also XviD codecs and an AC3 audio codec floating around, but I haven't found either to be necessary for any of the video files I routinely see.
Final note: I've noticed that a number of AVI files are in fact not AVI files at all, but Ogg Media (.ogm) files with the wrong extension added so they'll open in the default player on Windows or something. Quicktime does not know how to play .ogm files, but VLC does. If you have an AVI that Quicktime says is not a valid file (as opposed to just not playing the audio or video track), it is probably one of these. Again, VLC will play it, and if you get info on it you'll probably find that it's a misnamed OGM file.
