Shutter
This is a very powerful and feature-rich tool for taking screenshots. Written in GTK and blending well in GNOME, Shutter offers just about anything you would ask from such an application: timer, screenshots of whole screen, windows, widgets, a quick and easy-to-use editor for fast retouching or pointing out certain aspects of the image, support for plugins, exporting/importing and saving to PNG, JPG or BMP. Definitely a winner in my opinion.