I've been working on building a DJ system and I couldn't find a controllable crossfader abstraction. I found a number of ways to fade from one signal to another over time, but none that could be hooked up to a MIDI or OSC control for easy back-and-forth realtime fading. So I built one.
It's a constant-power crossfade available in stereo and mono varieties (although obviously you can hook multiple crossfaders up to the same control if you need more channels than that). The control input is on a 0 to 1 scale, not a 0 to 127 scale, so be sure and adjust accordingly. It takes a "depth" input, which is roughly analogous to the "sharpness" control on adjustable crossfaders. The default depth is 1, which gives a smooth fade suitable for beat-matching and similar applications. People doing scratching may want to use a depth of 2, 3, 4 or maybe even 5 to get a quicker fade in at the edges and a fatter middle area where both tracks can be heard. This is useful for techniques like transform scratching and variations thereon where it's nice to get a large volume response with little movement.
I'll make help patches for them at some point, but if you read the comments, they explain things pretty well.