xUbuntu - my main operating system running on my Fujitsu Amilo laptop (Intel Centrino Duo, 2Gb RAM, 160 Gb HDD). My current version - 10.04.
GRML Linux - nice LiveCD for system recoveries and backups.
Debian GNU/Linux - another great distribution I used a lot between 2005-2008.
VirtualBox - superb virtualization tool. Unfortunately, useless with OpenBSD which barfs segfaults all the time when run from VirtualBox.
Windows Vista and 7 on my computer at work. Not much to say here.
XFCE - my preferred window manager. Lightweight and fast. Has all necessary functions & a great choice of themes.
KDE - use KDE4 ocasionally, mainly when feeling tired of XFCE. Run various KDE applications quite often from XFCE. That's why I use kUbuntu, not XUbuntu which has XFCE by default.
ION - robust window manager I used between 2005 and 2008. Fully keyboard driven and scriptable in LUA language. A bit difficult to use with windowed applications like GIMP though.
Konsole - great terminal emulator, a part of KDE. Transparency is a big advantage of KDE apps.
GNU Screen - amazing tool. I started using it in April 2009 and am now becoming increasingly addicted to it. There's no need now to use tabs in Konsole, because GNU screen offers multiwindowing capabilities in any terminal, even in text mode.
GNU bash - GNU Bourne Again Shell. As standard as they come. Adore it a lot.
GNU Emacs - more than just a text editor. "Can be thought of as a thermonuclear word processor". Most important Emacs add-on is org-mode which helps me a lot to stay more or less organized. I use Emacs mostly in text terminal, using -nw switch.
Kate - use it ocasionally, mainly from KDE and/or as a scratchpad.
OpenOffice.org Writer - for Word documents and stuff to be printed.
Firefox - my default web-browser I spend a lot of time in. I use a bunch of useful extensions with my Firefox: ColourTabs, Tree Style Tab, DownloadHelper, Live PageRank, Xmarks and StumbleUpon. My Firefox is skinned with LittleFox theme, which is designed for optimal screen usage.
W3M and Lynx - text-mode browsers I sometimes use with scripts to dump pages and grab hyperlinks.
Internet Explorer 6, 7, 8 - to test webpages.
Inkscape - vector graphics editor, my primary tool to create new designs and slice them.
GIMP - a nice tool for raster image manipulations.
qiv - my default image viewer.