PC Status and Information: Gnome has a number of built-in stuff for this. System Monitor and the Sensors Applet are the ones I use the most
Security: My Buffalo hardware firewall and ClamAV and F-Prot to check files before I pass them along to my poor Windows-running friends/family.
Utilities: Too many to mention. My OS is essentially a collection of utilities:
Browsers: Firefox (also have Opera and Epiphany for emergencies)
[scott@desktop ~]$ pkg_info | wc -l
Audio: mplayer, Audacity, Rezound, Glame, mhwaveedit, Sound Juicer, Grip, Sweep, Audacious, Rhythmbox, Amarok, BMPx, Exaile, RealPlayer, Asunder
Video: mplayer, Xine, Totem, Ogle, RealPlayer, Kbtv, fxtv, xawtv
Media Encoding: Haven't been doing this but I could present a list of options if someone was interested.
Documentation and Inventory: Just installed GCstar
Web page creation: Kompozer
Graphics editing: Usually Gimp but sometimes Inkscape or Xara depending on the image. I also have Blender but I don't use it.
Usenet newsgroups: Pan
Word processing: Usually OpenOffice, sometimes AbiWord
Spreadsheet: Usually OpenOffice, sometimes Gnumeric
World exploration: Google Earth
Virtual machine: QEMU, DOSBox