My favorite one's is FastStone Capture. 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.