To add some more:
VLC - excellent video player, plays nearly anything you throw at it.
Evolution - groupware suite like Outlook... in case you don't like something about Thunderbird
amaroK - excellent audio player, for me, it's bettery than anything I have ever used on any os except foobar2000
Evince & KPDF - PDF viewers and more, a lot less bloated and nearly as functional as Adobe's reader
Hopefully others will mention their favourites too, since it's quite possible that EcPercy and me have forgotten some important stuff
