It's gotta be GTK-Gnutella. It's just such a perfect tool for nerds, and it's also fairly easy to use for newbies. I absolutely _love_ its GUI. While it may look a little complex and unwieldy to your average Joe XP user, once you get used to it, you'll find that it's an extremely powerful user interface, and you'll never go back to anything else.

I'm a die-hard Qt and KDE fan too -- I literally keep a copy of GTK+ installed just for GTK-Gnutella. Keep away from the GTK2 version for now, though -- it's basically just a memory leak in a box, and not yet ready for primetime. Stick with GTK1, it's excellent.