From Mirek Wojtowicz:
MWSnap is a small yet powerful Windows program for snapping (capturing) images from selected parts of the screen. 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 five 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.