All Linux and Ubuntu related, of course. And all free, of course.
PureData: This is the bomb for creating your own audio and visual applications. It rocks more than Stonehenge!
Inkscape: Classy drawing program, fantastic for logos and posters.
The Gimp: Photoshop® for the masses (as in a few hundred quid cheaper ie Free). All new version 2.6 now comes with more Window™-esque windows, so easier for newbies.
Scribus: I haven't done much in the way of print of late, but if I do, this is the program I use. The functinality is equivalent to Quark 3.11, if not actually better! Runs on Windows as well.
Pidgin: Do you have any friends? Online? If so, chat to them in Pidgin! Supports most chat protocols such as MSN, Yahoo!, IRC, Jabber (Google) etc.
Freemind: Very easy to use brain-storming tool which I use for creating and exporting site maps. Also is cross-platform.
Grsync: Do you need to sync and backup files? Then use this!
KeePassX: Do you want to keep your passwords secure and in one place? KeePassX does the job!
Audacity: Audio wave editor, simple, multi-channel, free. Magnificent.
gparted: Sorts out partitioning and formatting of disks.
Hamster: An applet time tracker, both easy to use and useful.