This article lists must-have desktop apps for Ubuntu such as amaroK (sync to your iPod), CheckGmail, Automatix, Gnome Deskbar Applet, & VLC.
Of notable interest is Beagle:
Beagle is the Spotlight/Google Desktop search app for Linux. However, think more Spotlight than Google Desktop (which is a good thing, because Google Desktop can suck at times). Beagle indexes your files and lets you construct quick, powerful file searches.
Read full list of tips at Top 10 Ubuntu apps and tweaks - Lifehacker