MPlayerWii is a native port of the MPlayer media player. It will play almost any kind of audio file but still needs improvement on video playback.

Place video and audio files on the root folder of your SD card.

Changes:

added USB storage support (USB reading is dead slow unfortunately, only good for MP3 files)
added volume controls (Up/Down buttons on the wiimote)
added wiimote buttons repeat during playback
added preliminary test network streaming (shoutcast and mms)
longer filename display in the file selection screen
fix for widescreen aspect ratio (uses the Wii display settings)
more audio playback fixes

http://www.wiibrew.org/wiki/Homebrew_apps/MPlayerWii