A number of linux applications and tools are already extremely useful. On my installation of Ubuntu 9.04, some of my most used applications are Gnome-DO, Pidgin, and Open Office. Their built-in functionality is great, but you can go even further! Here are some links to find out about really useful extensions and plugins for these three tools.
Gnome-DO: 8 Most useful plugins for Gnome-DO
Pidgin: 10 useful Plugins To Spice Up Your Pidgin
Open Office: 8 Essential OpenOffice Extensions
Hopefully you can put these to good use!
Enjoy!