Screenshot? Shutter at the Thought

Formerly known as GScrot, Shutter is a fantastic screenshot tool for Linux.
In their own words,

Shutter is a feature-rich screenshot program. You can take a screenshot of a specific area, window, your whole screen, or even of a website - apply different effects to it, draw on it to highlight points, and then upload to an image hosting site, all within one window. Shutter is free, open-source, and licensed under GPL v3.

For what it's worth, Shutter comes highly recommended by me. I am currently using it for all my screen capture needs.

Downloads are available for multiple distributions, of course, including Ubuntu. A quick deb install and your all set.