Slackware. Just before I started college I was sent the list of baseline requirements for comp.sci classes. Windows 95 or Windows NT, Visual C++, and a serial connection. I didn't have the money for '95 or NT; I was still using an 80486 with four (just before moving on campus, I traded up to eight) megs of RAM and wasn't in a position to get a new box (though I did drop pretty much my entire discretionary budget for the next two years into a one gig hard drive, which got me all the way through undergrad). However, there was a BBS in my NPA called Monolith, which was basically a Slackware Linux box with two dialup lines running homebrew BBS software. The sysop let me download the boot, root, A, D, and N disk sets (one floppy at a time - it took weeks) and helped me set up a basic Slackware machine. Once I got up to school I was able to set up a serial connection (and later, talk the building into lighting up my floor's ethernet lines). The rest, as they say, is history.