Greenshot

Greenshot is a lightweight, open source, and portable screen-capture tool. The executable and supporting .dll files weigh in at 464KB, making Greenshot a rather lightweight tool and well suited for a flash-drive toolkit. You can generate screen captures by a variety of methods including using the print-screen button as a hot key and activating the main menu from the system tray where you can capture by region, freehand cropping, etc. Greenshot can be set to send your crops directly to a printer, open for editing, or to be saved.

Once you have captured an image you can add text and shapes such as arrows to draw attention or a bright red box to isolate. One curiously lacking feature is the ability to crop after the initial capture. You can use the freehand-capture tool to carefully grab the area you want when getting the initial capture, but once you are in the actual editing stage there appears to be no method to crop.