browserling - Live interactive cross-browser testing from your browser.
Screenshot Machine - Screenshot machine is an online website capturing service. Creates a screenshot or thumbnail of any online web page in couple of seconds for free.
BrowserStack - BrowserStack is a software testing platform for developers to comprehensively test websites and mobile applications for quality.
URL2PNG - Screenshots as a Service. Capture snapshots of any website, right in your app, quickly and reliably.
Browsershots - Browsershots makes screenshots of your web design in different browsers.
ApiFlash - ApiFlash is a powerful serverless screenshot API built with Chromium and AWS Lambda. It can easily scale to millions of screenshots per day and has an ever growing number of satisfied big clients.