Wow - brings back memories ;) I think it was 1993 at the University of Canterbury in New Zealand. A stage 2 computer science source said we could use the lab for programming, or we could install this thing called Slackware Linux that had gcc and everything we needed for out projects. So, of course I did. Mmmm, floppy installs. So, it was mainly used early on for COSC assignments. A few years later setup Red Hat as a DNS and webserver in 1996 for our fledgling web development company. In later years after I stepped out of the IT field continued to use it for servers in our small business, although starting to fix it up with OS X as well. Never used it as a desktop - have primarily been Windows, and over the last nearly 5 years OS X (which seems to be a fantastic compromise).
But yeah, the first thing was COSC programming assignments in '93.
