No matter what your purpose is, when it comes to taking screenshots, Shutter is the only option. It is incredibly feature-rich and very fast – unless you enable the timer mode, of course. You can capture certain areas, individual windows, the entire desktop or even menus and tooltips.
Shutter isn't limited to just capture either, once the shot is taken, it can be transferred directly into the editor where you can add shapes or text, censor private information or crop some more if you got it wrong the first time. From there, you can even upload directly to a number of image hosts without leaving Shutter.