livecoding is an interactive sketchpad. Code modifications are instantly displayed - no need to refresh your browser. Click a number, adjust its value via the popup slider, and watch your work change on the fly!
100% totally based on Bret Victor's Inventing on Principle talk, which is one of the best I've ever seen. If you watch only one talk this year, make sure it's this one.
See the wiki for more information.
Also check out the Google Group.