Distros: 
Red Hat Linux 7/9
Red Hat Enterprise Linux 3/4/5 (RHCE 5 in progress)
Oracle Enterprise Linux 4/5 (AKA Unbreakable Linux, re-branded RHEL)
CentOS 4/5

Fedora (previously Fedora Core) 1/2/3/4/5/8/9/10

Ubuntu and Ubuntu Server

Since 6.06 LTS Dapper Drake, all the way through 7.04, 7.10, 8.04, 8.10, 9.04, currently on 9.10 Karmic Koala. (Running on my desktops and laptops, Ubuntu Certified Professional test in progress)

Keep It Simple and Stupid, Slackware and BSD like distro. Famous for its simplicity, user-centric, performance, ABS, AUR, aggressive upgrade strategy and excellent pacman package management tool.
What makes Arch Linux popular among Linux gurus?
pacman
yaourt (AUR front-end, script)
BSD-Style init (All in /etc/rc.d, you hate SysVinit? Give Arch a shot.)
AUR
ABS (ports like package build system)
Openness (assemble the system from base and keep it slim)
Last but not least, Arch is a Rolling Release

Debian GNU/Linux 3.0 woody/3.1 sarge/4.0 etch/5.0 lenny (stable for server) squeeze/sid for desktop.

Gentoo Linux Everything from source, the portage package management is similar to FreeBSD.
Slackware Linux 10.0/12.2 (KISS, most BSD like)
Mandrake(Mandriva) Linux 8.1/10
Linux Mint 6
Magic Linux 1.0
SUSE Linux Enterprise Server (SLES) 9/10
Optware (embedded Linux for NAS or things like that, ipkg package management inspired by dpkg and APT)

Live CD/USB tool distros:
BackTrack (U know what it is;-))
CloneZilla Live
SystemRescueCD
Knoppix (Live CD based on Debian)
GParted Live CD/Parted Magic
Slax (Live CD based on Slackware)
Puppy Linux (on USB flash disk, built based on Slackware by an Australian)
Damn Small Linux

Desktop Environment: GNOME, XFCE, KDE, LXDE
Window Manager: Enlightenment (E16, E17), Fluxbox, Openbox, awesome, FVWM, enlightenment, Window Maker

I have tried almost all famous distributions on DistroWatch. Well, obviously I can't use all of them at the same time.

To sum up, my favourite distros are Arch Linux and Ubuntu. I do like Gentoo but I still don't have the patience to compile everything from source. Last one, Debian GNU/Linux is a very good choice for web server (stable), sid for testing or development.  I use Ubuntu and Arch Linux on my D620/D630 both at work and home everyday.

I also use Debian stable (lenny now) and Ubuntu Server to host Wordpress, Mediawiki, Confluence.

Oracle Enterprise Linux to test commercial products, troubleshoot and portal performance tuning, middleware and database environment, as well as POCs(proof of concept).

Active memeber of Shanghai Linux User Group (SHLUG) and Sydney Linux User Group (SLUG).