MWSnap

Current version is capable of capturing the whole desktop, a highlighted window, an active menu, a control, or a fixed or free rectangular part of the screen. MWSnap handles 5 most popular graphics formats and contains several graphical tools: a zoom, a ruler, a color picker and a window spy. It can be also used as a fast picture viewer or converter.

MWSnap does not require installation and does not need any special dlls, drivers or system files which can mess up your system.

http://www.mirekw.com/winfreeware/mwsnap.html