SnapaShot captures the entire screen or parts of it in a slightly different way. The interface provides a transparent framework that can be changed in size by stretching their edges. Simply move it across the screen and "snap" the part that you want to save. After cropping it, you can copy it to the clipboard or save it directly to a file.
What I really like about this program is that it doesn't require installation. It can be run directly from the EXE file including from a USB stick. Unfortunately, you get limited control over how you shape and add effects to the screenshots afterwards although if you upgrade to the full version, you' get a lot more.
As far as lightweight screenshot capture tools go, this is one of the quickest and smallest you'll find at just 64KB in size.