BuiltWith and the BuiltWith Chrome are fantastic tools that allow the user to see all technologies used to build the frameworks of a website. When qualifying potential clients, I use this tool to see the platform they are currently built up on.
The option to use either the full-fledged web app or the browser extension makes for a great user experience.
Being able to build a list of prospective clients based on technologies used is a feature I've only just discovered, but I'm now using regularly. Review collected by and hosted on G2.com.