Wappalyzer is a free browser extension which detects and displays the technologies used on websites: servers, content management systems, eCommerce platforms, analytics tools, advertising frameworks and more.
The extension adds an icon to your address bar, and clicking this displays whatever Wappalyzer has spotted, sometimes with their version numbers.
Clicking the name of any technology displays some statistics about it: the total number of sites where it's been detected, the top ten of those sites, how its use has varied over time.
If you're thinking that means Wappalyzer must send its detection results back to the developer, then you're right. This is done anonymously, though, so doesn't constitute any significant privacy risk, and if you're still unhappy then it's easy to disable (click the icon, select Settings, clear "Anonymously send reports...").
A handy tool for web developers, penetration testers, and anyone else who needs to know low-level server details