List of free screenshots stuff for developerππ½
If you're looking for all programming tools around screenshots (not just free). Check this page List of screenshots Programming Tools

ScreenshotOne
Fast and reliable screenshot API built to handle millions of screenshots a month.
Tag: screenshots
100 screenshots 20 requests per minute

Refero - design references for your next project
The biggest collection of design references from great web applications
Tag: design, inspirations, web apps, screenshots
Free tier after sign up

Puppeteer
Most things that you can do manually in the browser can be done using Puppeteer
Tag: screenshots, ui-testing, crawl