GAIM, the instant messenger client.
amarok, best music application I've ever used.
k3b, best cd/dvd burning application I've ever used.
Kate, a kick-ass graphical text editor. Code folding, saved sessions, embedded terminal, split viewports, it rocks. And I'm liking the idea of having the tabs vertical rather than horizontal in a text editor. Another kick-ass KDE app..
Ok, now I'm using Kopete, in GNOME, it's a fucking brill IM client, Pidgin (ex-GAIM) has nothing on it. Additionally Pidgin owns GAIM big time.
Sine now essentially 100% of the apps I consider best-ever (on any platform, and free or non-free) are KDE apps, I'm installing KDE (kubuntu-desktop) and checking it out. Probably won't be arsed to setup my panel and all again, though, and will stick with GNOME. It's great how the different DE apps work great outside their own DE. Freedesktop ftmfw.
Oh actually, the GNOME document reader evince is very, very good too.
Azureus, is an awesome BitTorrnet client, too.
edit: oh, and I simply MUST sing more praise for Amarok. I been using it to listen to some internet radio, and fucking sweet it is. Discovering so much cool music! This is what I love about so many of these KDE apps, they seem so focused on making things I'd usually never look at so simple - just sticking them in my face (yet, you can stay away if you wish), so that I use them, and fucking love them. I've also been using Amarok to manage my mother's ipod shuffle - and it's the bomb, the best. I must get on my brothers mac book pro sometime and see if iTunes can touch Amarok these days.