Stuff that I insist on regardless of platform (that is, I install these even onto Windows systems if I'm forced to use them):
- Pale Moon (web browser)
- Claws Mail
- GIMP
- vbindiff (command-line hex editor + diff utility for binary files)
- mercurial
- perl
Stuff that I require only on Linux systems for desktop use:
- Pan (yes, really, I still use a Usenet newsreader on a daily basis)
- qemu
- conky
- Aqualung (music player—I like odd software)
- Inkscape
- Scribus
- PySol ;)
- rdesktop (less a favourite than a regrettable necessity)
- various TDE built-ins: konqueror (as file manager only), kedit, kate, konsole, ark