ShareX - ShareX is a free and open source program that lets you capture or record any area of your screen...
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.
Thum.io - A real-time website screenshot generator
URL2PNG - Screenshots as a Service. Capture snapshots of any website, right in your app, quickly and reliably.
Screenshot Bin - Generate website screenshots and thumbnails at scale, right from your app. Screenshots as a service.
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.