Wappalyzer is a browser extension that uncovers the technologies used on websites. It detects content management systems, eCommerce platforms, web servers, JavaScript frameworks, analytics tools and many more.
Now called Wappalyzer, I think this is one of my most used chrome extensions. I am still a big fan of BuiltWith for extensive analyzing of a site's technologies, but from a UX and quick scan, Wappalyzer is by far the best.