SIPB stands for the "Student Information Processing Board". It was founded in the late 60's and has been a meeting place for it's particular breed of nerd for a couple decades now. In a lot of ways, SIPB is a social club, like a fraternity. We even have "Sigma Pi Beta" jerseys as a spoof on the ubiquitous MIT frat shirts. By hanging out in SIPB, I learned more about computers and networking and nerds than I did in any of my classes, and I often learned a lot in my classes.
I started hanging around SIPB freshman year and was not really much of a computer geek. My family had an old IBM AT, or maybe it was an XT. I don't really remember, which shows how much I was into it. I had a Commodore 64 when I was younger, and loved that. I wrote a lot of BASIC programs.
Typically spending time in SIPB involved hanging out there for long afternoons asking the older members questions and playing with random stuff, trying to get a program to do something, to learn a new language, or to make something work for the MIT community.