My favorite one's is [FastStone Capture][1]. I used it for several years.

It provide different screenshot mode:

 - All your screen
 - The active window only
 - Detection of your opened windows if you want to screen a specific one
 - Select the screen shot zone (rectangular or free zone)
 - A scroll-bar support allow you to screen a big page with 1 screenshot

You can automatically send your screenshot to:

 - The **editor**
 - The clipboard
 - Your printer
 - A file (auto-save or not with naming customization, file format, ...)

And of course you can override the *print screen* key comportment.

To conclude the soft is free and very light.

  [1]: http://www.faststone.org/FSCaptureDetail.htm