Yesterday I made a Greasemonkey script to detect telephone numbers in hCards and wrap them in callto: links to launch VOIP tools (e.g. Skype). This is the kind of thing I do to satisfy my own curiosity, assuming no one will ever use. Another such project was the Google hCalendar Greasemonkey script I did a while back, except I did that one in exchange for a free book. And it's now being used by Yahoo.