I just found a wonderful keyboard shortcut. Press Ctrl+Shift+PrtScr and the mouse cursor will turn into a cross which allows you to mark a rectangle of the screen (by left click-hold, drag, release -- you will see a visual rectangle) and it will save that part of the screen into the copy&paste buffer.
Now, while that is very useful, I can't seem to figure out which program is responsible for this feature. My question is: how do I find which mechanism (software) is catching my keystroke?
I am running Ubuntu 12.04 LTS.