Full Page Screen Capture - I built a Chrome Extension that allows you to save a screenshot of an entire webpage as an image
Placeit - Generate realistic product shots in seconds
Webshot - Free and Open Source screenshot website service
AppLaunchpad - Create free beautiful screenshots for App store/Google play
Geekflare Screenshot API - Take a full page screenshot of any site from multiple locations whether it be via desktop, mobile or tablet
Smartmockups - Create stunning product mockups, easily and online.