KDE2 on Mandrake 8 for me. The look and feel of that desktop was a big part of what made me want to start using it over Windows. Whereas XP had just launched the Bozo the Clown-inspired theme, KDE offered something that somehow looked more real, and lived-in.

The other part, of course, was that the tools it shipped with were far and away better than the ones on Windows. Konqueror (way faster than Netscape) and Gaim (way more functional than AIM - I know that wasn't part of KDE) are two that came to mind almost immediately. Nowadays I live my life in a Web browser, and I'm not sure I'd have had the same impressions there.