1. Since his father's dental practice was operated from their home, he built a software program he called "ZuckNet" that allowed all the computers between the house and dental office to communicate with each other. It is considered a "primitive" version of AOL's Instant Messenger, which came out the following year.
2. In his sophomore year, he wrote a program he called CourseMatch, which allowed users to make class selection decisions based on the choices of other students and also to help them form study groups.
3. And who can forget the famous FACEMASH.