The VJ software: RaveGear Multimedia Juicer lets you mix digital video files and effect them in real-time. 

I have tried both the beta and the new version, and personally found the beta to be a bit easier to figure out, given that everything is in Japanese.  The new version however adds a host of new real-time video manipulation effects, including: roto-zooming, video warping & innovative text effects.  All of the these pictures are from the beta version of the program.

New Version Features:
-Drag and Drop usage for video loops.
-3 simultaneous layers possible.
-Video scratching, with speed control, fading, and time warp fx to each separate layer.
-Output can be saved as a Quicktime movie file.
-Video FX macros now available, along with 200 index point for video files with choreographed settings and effects for each point.

These shots are from the new version of the software.

They also offer a 3 CD series of VJ loops as an adjunct to the program.