I had been using mPlayer_CE, but i hear there's new app in town called WiiMC:
http://www.wiimc.org/
mPlayer CE doesn't have DVD menu support, while WiiMC says it does.
Pfff. Nintendo owned this years E3!
Check out their lineup:
Epic Mickey!
NBA JAM!
Donkey Kong Country Returns!
Kirby's Epic Yarn!
Metroid: Other M!
The Legend of Zelda: Skyward Sword!
Now THAT'S what i'm talking about.