I'd been using VLC in Panther seemed reasonably buggy in Tiger so I started using MPLayer OS X seemed okay, then I saw someone mention XinePlayer so I gave that a go, it's my current Tiger media player of choice, it works with heaps of codecs (all 3 seem to handle the exact same codecs) but ymmv with Windows media, but then you could just install WMP for Mac?
I'd of posted links for all of these but it's been a long day at work and GIYF.
VLC imo is the best program for most formats out there, with the exception of WMV3 which can be played only using Windows Media Player. Also, VLC was a little buggy in Tiger (version 0.8.1) but now it has been updated to 0.8.2 beta2 and runs fine, at least in my computer. I wouls strongly suggest this player.