BuiltWith scans websites and reports on the technologies they're using. Enter a domain, and you'll see the content management system, analytics platforms, advertising networks, JavaScript libraries, web servers, and e-commerce tools that site relies on. It can also show you which sites use a specific technology, which is useful if you're trying to figure out what's popular or looking for examples of a tool in the wild.

The free lookup gives you a summary, while paid plans provide more detail, historical data, and lists of sites by technology. Sales teams use it to find potential customers, developers use it for competitive research, and marketers use it to understand the tech stack behind a competitor's site. It's a niche tool, but when you need it, nothing else does quite the same thing.