I use Ubuntu right now. It has the best support forums and frequent updates, and is the best distro I've used over the years despite some glaring bugs.
The most common apps I use are Firefox, gedit, Audacious, the shell, Brasero, VMWare (for testing Exchange setups in Windows), AbiWord, GIMP, Asunder, VLC and Pidgin.
I started out on Red Hat 5.0 (not RHEL) in the late 90s. Used it full-time for about a year, then switched to BeOS and dual-booting that with Windows. Over the years, I've also used Caldera, Corel Linux, Mandrake (before it was Mandriva), SUSE and some others I am sure I'm forgetting.
My desktop is Gnome, though it's not nearly as customizable as I'd like (used to be, though, before the developers removed it all). I don't care for KDE that much, though.
I'm going to try Fedora Core 11 when it comes out, but will probably stick with Ubuntu, I'm guessing.