Wiring for Processing
Wiring for Processing is Wiring firmware and a Processing library that allow you to use the microcontroller from the PC. That is, you can read sensors, detect buttons presses, control LEDs, turn on DC motors, and the like straight from Processing by downloading a single piece of code to the Wiring board. Check it out.
Screenshot from an example Wiring for Processing program.
It didn't take long to port this to the beta version of Processing and package it up into a library, but documenting took a while. I like the new input example (with the abstract Wiring board) a lot, though.