Personally, I really like MWSnap 3.0.
You can capture the entire screen or any part you decide to select. Here are some (but not all) of the features:
5 snapping modes.
Support for BMP, JPG, TIFF, PNG and GIF formats, with selected color depth and quality settings.
Auto-start with Windows.
Minimizing to system tray.
An auto-extending list of fixed sizes, perfect for snapping images for icons and glyphs.
A zoom tool for magnifying selected parts of the screen.
A ruler tool for measuring screen objects lengths.
A color picker showing screen colors with separated RGB parts.
Fast picture viewer.
Adding frames and mouse pointer images.
Multilevel configurable undo and redo.
Configurable user interface.
8 years of release and upgrades.
I don't think you can do any better, even if you BUY one for big bucks.