MP3 - .mp3 - QuickTime Player, iTunes, VLC, etc. Just about anything will play MP3s now. My personal favorite is iTunes 4.
AAC/MP4 Audio - .aac, .mp4, .m4p - iTunes will play most of these audio files, QuickTime Player will also play these AAC/MP4 files.
MPEG-4 Video - QuickTime Player 6 and up will play MPEG-4 video files.
OGG - .ogg - VLC is your best bet. Mplayer may also work.
Windows Media - .wma, .wmv, .avi, etc. - For non-MS apps, VLC should handle most WM files. There are a few that don't work though. In that case, you will need Flip4Mac.
FLAC - .flac - This app will decode the .flacs into MP3s or AIFFs. MacAmp X Lite and the FLAC plug-in will allow you to play the files the way they are.
Shorten - .shn - Shorten X will convert the .shns into .WAV files which can then be converted into MP3s or AACs using iTunes. MacAmp X Lite and the Shorten plug-in will allow you to play the files the way they are.
(e.g., the use of video cameras with Macs for importing / editing / exporting video footage)
Although somewhat dated with its information on overall pricing, 3rd-party video products, and not listing what could be Today's used Mac (this was written 1999/2001), please check out Dennis R. Dimick's article: