Under Quicktime with the help of plugins such as DivX and Flip4Mac you can play:
AVI (DivX, Xvid, h.263, h.264, indeo, just about anything else you can throw at it), MPG, MPEG Layer 2, MPEG Layer 4, WMV, ASF, 3vix, MOV (of course), um yea I can't think of a single video format I'm not able to play under Quicktime.
Then there's Flash video that seems to be popular in websites these days
There's Real Video of course
Thinking of getting a Mac? I switched from a PC almost one year ago today, and so far have not regretted it whatsoever. Boot Camp helps a bundle when I just miss my PC games.
2009 iMac 21.5", iPhone 5S, too many iPads
Are you backed up??? If so, why? Psyllium Fibre is not free but will do wonders!
VLC can do just about anything .. there are a few formats it doesn't handle (like some of the proprietary WMV type stuff).
dona83: I can think of a few formats that quicktime doesn't play (especially without having to pay $$ for extra codecs)... but VLC does for the most part and it's free. fourcc is the one format that comes to mind that VLC doesn't do on OSX though