Wii Media Center X is basically a "free multimedia server developed by Red Kawa that allows you to stream certain types of pictures, music, videos and files from your computer (where the server running) to your Nintendo Wii. It runs on most major operating systems including Windows, Mac OS X and Linux." Download page. Video demonstration after the jump.
Wii Media Center X lets you stream and watch videos in the Flash video format (FLV) on your Wii. You can download flash videos from many places on the internet or create your own using Wii Video 9...lets you playback MP3s on your Wii without the need of an SD card