i would NEVER download copyrighted material, of course. but the "home movies" i watch on my computer usualy go through media player. lately, i tend to play divx through the divx player. the post proccessing effects (if you have a r3xx based card) clean up the image a bit, and the player is a bunch snapier than it used to. some movies i play with microDVD if they require subtitles, plus it's fun to make custom menus and chapters. if you want a good, general purpose, multimonitor capable player (and don't like media player), try zoomplayer.
c:
ps-these are all windows players. i never play movies on my mac or *nix based systems