Greenshot - Greenshot is a free and open source screenshot tool that allows annotation and highlighting using the built-in image editor.
Ape Mockups - A huge hand-curated collection of free high-quality mockups
Shutter - Shutter is a feature-rich screenshot program.
AppLaunchpad - Create free beautiful screenshots for App store/Google play
Snipping Tool - Use Snipping Tool to capture a screen shot, or snip, of any object on your screen, and then annotate, save, or share the image
Clay Mockups - Free set of beautiful device mockups for your designs