CircleMUD was my first intro to C and network programming. I was at a computer camp for 8-10 year olds that taught BASIC. One of the counsellors was doing their undergrad at caltech and pulled the students who were bored by BASIC into a separate group to learn C by editing CircleMUD.

Hacking up a pre-existing MUD was the perfect mix of immediacy / interactivity and fun to hook me. So thankful to that guy, wherever he is. He probably changed the course of my life.