CoScripter is a fantastic tool. I really hope we can find a way to bring it to the mass market, both because it's cool and useful, and because it leverages the "open box" nature of the Web, the same openness that allows Greasemonkey to exist. That openness is good and powerful, and a strong technical advantage that the Web has over its competitors.
I want to mention that this work originated with Rob Miller and his students
Greg Little and Michael Bolin at MIT with their development of Chickenfoot. Greg developed Koala/CoScripter while an intern at IBM Almaden.
It's especially interesting because I've known Rob for a long time --- I remember talking to him when he was a prospective graduate student visiting CMU, back in 1995. I've also known Tessa, an IBM author on the Koala paper, for several years. Small world eh!