Shutter is probably the most powerful screenshot application for Linux, and the main reason for this is that it comes with tons of configuration options for the final process of taking a simple screenshot. And why not, considering there are people out there who need to take a screenshot of a single window or a desktop region instead of fullscreen only, like the GNOME default screenshot program. Of course, there is KSnapshot which offers these two options too, but that's where similarities stop.
Shutter 0.86.4 in Ubuntu Maverick Meerkat Beta
Shutter has much more to offer than the usual screenshot program. It allows you to take the screenshot of the fullscreen, selection (press Enter after selecting the rectangular area to take the screenshot), window, menu, tooltip, and it also provides a changeable delay from when you give the command until the actual screenshot is taken. You can print the image on-the-fly directly from shutter, without the need to open an external image viewer.