Here are some of the best Wiibrew media players I've encountered. They all have their share of bugs and limitations, but they also have their bright spots.
First is DragonMedia Player. It supports a number of audio files and playlist types, and can read from the Front SD and USB. Unfortunately it doesn't have support for video, but at least it's skinnable.
DragonMedia Player 0.14 Beta
Next is MPlayerWii. Nice audio player, and has some support for video playback. Lags sometimes though.
Last is GeeXboX. Like MPlayerWii, it's based on the open-source MPlayer for Linux, Windows, and the Mac OS X. It supports all of the audio and video formats that MPlayer does - that includes RMVB. Pretty nice player.
You guys have any Wiibrew Players you use?