Knoppix. I didn't see it listed yet so I had to chime in.
I saw it and was confused that computers could run something that wasn't Windows and wasn't Mac. Then I was handed a Knoppix LiveCD and suddenly MY computer was Linux. Absolutely blew my mind.
I then explored Mandrake (now Mandrivia?) for a while but it never really stuck.
A few years later Ubuntu was handing out LivdCDs to everyone running Warty Warthog and soon after window managers started to use Beryl (?) which let you have a fancy cube desktop. Absolutely pointless but that's how it all started.