Catching up on a major backlog of e-mail reading I came across the headline in the Slashdot daily mailing about Slackware's tenth anniversary. It was fun to read all of the comments and see how many people had used Slackware as their first distribution.
When I first started college in 1997 I was given an account on a FreeBSD machine. Of course I just had to know more about how to use it, so I bought the book and CD set from Walnut Creek CD-ROM.
Not very long after, a friend of mine purchased RedHat 5.1 from BestBuy and we played around with that for a while. I was taking a UNIX class in school at the time and was able to use what I learned in class on my FreeBSD system or his RedHat Linux system.
That summer (1998) I took an advanced UNIX class in which we set up our own machines with copies of Slackware (3.0, I think). From there I was hooked.
I have occasionally tried other distributions, but have always come back to Slackware.