So is Media Player.
Actually Media Player is a different story to the other two. MPlayer really is just a player. It is a 13meg player (and that's just the install file). You can download the codec pack seperately in 900k. Media Player is not smart enough to play anything that isn't covered by a codec, except of course the windows media format which is supported via dlls. But that comes in the codec pack, seperately to the player itself. It is possible to use windows media 9 codecs on media player 7 and above...
I actually find both MP and Real are more reliable than QT on my pc,
I've seen direct comparisons of the three formats and I must say Real are showing their age. In a low bitrate stream the colouring in a realmovie file is atrocious. Other formats just got spotty/blocky. QT is a little blockier than MP, which is my personal fav of the three, at least on Windows. But, I believe the world should install XviD and an ogg codec and everything would be peachy =]
None of them force you to use their player, but no other player will play their particular format.
Any decent player on Windows will support windows media format out of the box. It is completely free for them to do so. I'm not sure about QT (so long as the player is installed) and I'm pretty sure Real keep very tight lipped on their technologies. Nevertheless, Winamp has plugins that can enable all three. So it's not impossible.