What are your must-have apps?
fsilva: For linux users, a very useful app: BasKet Note Pads. 
It's an organization tool, you write pages with notes, format text in those notes, dragndrop files into the page, and the killer feature: at any time you can capture a part of the screen and it goes directly to that page. (e.g. you can be writing notes about a PDF and capture text/a graph/an image from it, directly, in a second)
I can't live without it. Out of curiosity, does anyone know similar apps? (in any OS)link: http://basket.kde.org/