ShareX

Website: http://code.google.com/p/sharex/

Description: ShareX is an open-source program that lets you take screenshots, save them in your clipboard, hard disk or upload them to over 20+ different remote locations. ShareX can capture screenshots with different shapes: rounded rectangle, ellipse, triangle, diamond, polygon and also free hand. It can also upload images, text files and all other different file types. It is able to capture screenshots with transparency and shadow. The program also supports drag n drop, and clipboard upload.

Comments: It's an incredibly useful program, I use it on a daily basis.