Greenshot"... Recently, I was writing an article and needed to take some screenshot and do minimal editing to make instructions clearer.
Of course, Snag-it, from TechSmith is the leader and hands down the best tool for such things but it's not free and I could not justify buying it to myself as I rarely need to use it at home.
Greenshot is a free, open-source alternative, that did the job well. When you launch it, you will notice a new tray icon ( menu is shows on the left). It then detects when you hit "Print Screen" key in Windows, and allows you to specify the area you would like to capture. Once done, you can put annotations like some text, arrows, and shapes (circle some text, etc.) and save it as .png or .jpg (is anything else being used these days?)
That's it. If you are looking for an alternative on Windows, give it a shot.