Neuromixer Pro 0.5 is here for Windows XP and Mac OS X:

Load up video, and scratch it like vinyl. Neuromixer Pro
features playback speed, range/cue points, forward/reverse, dual video
"decks", and real-time MIDI-based manipulations. The new version
features:

• Support for Ms. Pinky (so you can scratch with real vinyl!)
• Feedback video effect
• BPM counter receives MIDI clock (so you can sync your video system to your audio system — bring two laptops, I say)
• Improved previews, MIDI, preferences, etc.

Still free for now, though summer 2005 will bring the US$49.95 1.0 release — likely to be well worth it!

And if you're Japanese and are tired of listening to me speaking English, VJ Fader translated his page into Japanese.