Annotate - Annotate (formerly know as Glui) is the simplest way to capture, annotate and share screenshots.
Greenshot - Greenshot is a free and open source screenshot tool that allows annotation and highlighting using the built-in image editor.
LabelMe Annotation Tool - LabelMe is an annotation tool writen in Javascript and Perl for online image labeling.
Shutter - Shutter is a feature-rich screenshot program.
Monosnap - Monosnap is a comprehensive screenshot taking software system that provides users with a fast and accessible way to take screenshots whenever they want. Read more about Monosnap.
MWSnap - MWSnap is basically a free to use Windows snapping tools that are used for snapping any part of the screen that is currently displaying on the front of all opened programs and windows.