It is not very often to see open source multimedia tools around for narrow niches.
Luis de Bethencourt (from Ubuntu Studio fame) just released the 0.2 version of Freemix, a live video mixing tool for VJs. It basically allows to mix short video clips live, setting the pitch, tempo and that sort of stuff that is quite common to see in DJ/VJ gigs nowadays.
Freemix is powered by Python, Gtk+ and GStreamer. Go and grab it while it’s hot!
If you are interested in contributing, make sure you poke Luis through the launchpad development site, the project needs some love to get to 1.0.