Operating System Stuff

Debian Linux
Very good Linux distro with a huge software repository. I switched to it from Slackware when I built my new 64-bit machine.

Slackware Linux
One of the oldest, and in my opinion the best, Linux distro around.

OpenBSD
Very secure version of BSD. I use it on my router.

LinuxQuestions.org
A great source of information about Linux and *BSD.

Fluxbox
My favorite X window manager.

Opera
A very good web browser.

Chromium
An even better web browser.

VirtualBox
Open source x86 virtual machine.

Programming Stuff

Ruby
My new favorite programming language.

Perl
Just about my favorite programming language.

Perl Monks
Perl forum and more.

Higher Order Perl
Excellent book on Perl. Somewhat advanced, focuses on using Perl to write Perl. Awesome.

Emacs
A flexible, programmable text editor.

GCC
The GCC compiler.

C++ FAQ
Every language should have a FAQ like this.

SGI's STL documentation

SBCL
The best open source Common Lisp compiler I've been able to find. Used it to play with SDL, OpenGL, and Lisp

Glasgow Haskell Compiler
The de-facto standard Haskell compiler. Used this to write some compiler-related stuff.

Journal of Object Technology
An online journal edited by one of my old professors. Focuses a lot on Java.

Graphics Stuff

Rhino3D
A high quality NURBs modeling/CAD program.

Wings3D
Pretty decent open source polygonal modeler. Not as good as Rhino, but a lot cheaper. Somewhat notable for being written in Erlang

3Delight
A high quality 3D rendering program compatible with the Renderman specification. The first license (for two processors) is free.

SoftImage
A very high quality 3D modeling/rendering program often used for computer graphics in games and movies. Unfortunately, it's very expensive.