Convert, transform, edit pretty much any video format into pretty much any other video format. You can open any .VOB file and save it as .MP4 or whatever. An awesome easy to use piece of open source software.
GAP - the GIMP Animation Package
Exract animations or videos into images or put images together into animations or videos. Very useful to create animated GIFs from videos.
One of my favorites, it enables me to watch TV over the laptop with the USB-DVB stick on all other computers all over my network.
MPD - the Music Player Daemon
Runs on the laptop connected to my stereo, so I can listen to music over it. I can control it from every other machine in my home. I also can stream the music to any machine I want.
I play my favorite Windows games using WINE. I use it at work, too.