Well, WMP9 does play DVD's, VCD and SVCD out of the box. For WMP 7.1 you need to have a decoder installed to play DVD's. For QuickTime..WMP9 plays QT content out of the box. It doesn't associate itself to QT files automatically though, you have to do that manually(for legal reasons probably). It plays DivX with the DivX codec installed and it will play Real Media with the Real Media codec installed(you have to go underground to find the RM codec though ;P).
All builds of WMP9 other than the newest one (2848) installs and works in Win9x and Win2K. I know the taskbar player doesn't work in Win9x/Win2K(it is a WinXP/.NET only feature). I don't know how well it plays other propierity formats, DVD, VCD, SVCD, etc. in Win9X/2K.
(EDIT)I have never used XMMS before so I can't comment on it's little player Window. Does XMMS also have Full Screen video controls that appear and dis-appear at will, does it have Tru-Bass and WoW Effects, does it have it's own Video Overlay controls(for people who have a video card that doesn't offer overlay controls in the drivers), does it have Speaker enhancments(like WMP9 does with XP Plus)?
[ August 22, 2002: Message edited by: Zombie9920 ]