Desktop related software. Most users interacting with computers use desktop software.  The software normally runs within a Desktop Environment that allows for the interaction of the software with the user.  There are several Desktop Environments to choose from.

Modelling

Wings 3d

A nice, fast, and simple to use 3d modeler available for Linux, Windows, and Mac OS X. I found the interface to be very intuitive and have built several models with it, including the dune buggy at right.

Browsers

Firefox

An excellent browser. Used frequently and works well.

Seamonkey

Formerly the Netscape suite, this includes a browser as well as a WYSIWYG (What You See Is What You Get) HTML editor that allows you to publish straight to a Web server.

Thunderbird

Evolution

KMail

Items marked with (nyt) indicate I have not yet tested it. I've heard
it's good (so it has made it on the list) but have not yet tested it
myself.

Development tools, including compilers, debuggers, development environments, version control, utilities, and modelling tools.