You can download emulators straight off the homebrew channel, I have tried the NES, SNES, and Genesis ones (there are about a half-dozen NES and SNES emus and I suppose they're all about the same) and they work really well, with decent options for control and such.
I haven't much use for the wii as any kind of media player, so I can't help you there. I don't even know why anyone would want to play DVDs or media files through the Wii.
The Quake portover you can download off the homebrew channel is awesome and works really well using the waggle as a pointer, but you need to copy the pak1.pak over from the full version of Quake to play the whole game - the downloadable version is just the shareware levels.
WiiDoom is buggy, and doesn't seem to work with any of the Doom wads I installed. The guy is still working on it, though.
I've only downloaded one homebrew game so far, BARRAGE, and I was bored with it after 30 seconds. I'll try some of the target-shooting games next.
I'm still messing around with the emulators, gonna try the N64 and Sega Saturn ones next, will let you know how they work.