graphics/ImageMagick or its newer version, or its fork graphics/GraphicsMagick. They may work slightly differently but the general idea is the same.
These are command-line toolkits (that do many things other than taking screenshots), but I'm sure linking one of their commands to some keyboard shortcut or menu item should be trivial. Maybe even create a script to display the screenshot in your favorite image viewer.
To take a snapshot of your screen simply run import /tmp/snap.png and drag the rectangle. And for a screenshot of the entire screen use import -window root /tmp/snap.png.
Criosphinx, thanks. It worked. After installation, I accessed the Screenshot tool from Applicantions --> Accessories.
Beastie, I tried GraphicsMagick, but there was some error in the installation via ports (some Perl library was not found). I'll keep both, ImageMagick and GraphicsMagick in mind. For the time being, the simple Screenshot tool worked for me.