The Virtual Theatre Project at Stanford University
http://ksl-web.stanford.edu/projects/cait/
The Oz Project at CMU
http://www-2.cs.cmu.edu/afs/cs.cmu.edu/project/oz/web/oz.html
The Media Research Laboratory at NYU
http://mrl.nyu.edu/projects/improv/index.html
Impro by Keith Johnstone
http://www.keithjohnstone.com/