Team Twiizers is at it again, releasing a Wii port of the MPlayer media player. If you’ve already added the homebrew channel, you just need to download an install a couple of files to get things working.
You can read more details about the port at HackMii and WiiBrew. There’s also a convenient instructable to hold your hand through the install. It seems silly that Nintendo didn’t release this as a built-in feature, but at least you can get it working on your own now.