I had further requirements:
* The tool have to be free;
* It should work on Windows 7;
* It should work at background;
* It should have hot-keys;
* It should save files in popular extensions, such as JPEG or PNG (hello, Snag);
* The process should be done with minimal amount of clicks.
So I downloaded some tools and started to rate them.
Here are tools, that are too complicated in the first step, so I unistalled them right after opening:
* Screenshot Captor
* Screen Grab Pro
* Screen Hunter
Here are tools, that passed the smoke test:
As you can see, all yeses have only one tool - PrtScr. So PrtScr it is!
Why PrtScr is cool:
a) I click "Print Scrn";
b) holding mouse right button I highlight something (+Ctrl if you want a rectangle);
c) with mouse left button I select the capture area (+Ctrl to rectangle);
d) I have a JPEG file in the specific place, without any edit or save buttons.
Example of the result:
Why PrtScr is not perfect:
There is no undo command: if you did 2 selections and the 3thd was incorrect, to fix that you should start all over again.