As a student, it was common that a course I wanted to enroll in was full, but often in the following days, students would drop the course as their scheule changed.  These spots didn't last long, and I found myself checking several courses several times a day to see if there were changes.

Near real-time course enrollment data is made available to the public, so I wrote an application that monitored the enrollment data and notified users of openings.  Users could also view the watch list in a simple calendar, which was not possible through the university course enrollment tools, and could search for courses based on open times they had in their schedule.