A whole bunch. When I was first introduced to Linux, I played with SLS for a bit (yep, I've used Linux before Linux was cool), then used a very early version of Slackware. After that, I used Red Hat and its variants for a while, then switched to Debian, which I used until I switched to Ubuntu, which I use today.

In work environments, I've been exposed to RHEL and SLES distros, as well as Mandrake and SUSE Desktop Linux.

You might say I've been around the block a few times.