Today I implemented emulation for the Wacom Touchpad. The idea behind implementing this was that many remote display protocols wish to deal with the mouse in absolute coordinates whereas PS/2 mice report in relative coordinates. Stephen Tweedie had the brillant idea at the recent XenSummit of just emulating a Touchscreen instead of a PS/2 mouse which solves this problem since Touchpads use absolute coordinates.
After milling around, I finally got around to it today. All in all, it took a few hours going from zero knowledge to a functional emulated Wacom device. Having an X driver really helped. I talked a bit to some folks in IRC about how to expose it in QEmu so now I just have to throw a patch together.
Without the source for X, this would have easily taken me a couple weeks to do...