Netcraft is an Internet services company based in Bath, England, which provides webserver and webhosting market-share analysis, including web server and operating system detection. In some cases, depending on the queried server’s operating system, their service is able to monitor uptimes; uptime performance monitoring is a commonly used factor in determining the reliability of a web hosting provider.

Their “What’s that site running?…” service is a very easy way to quickly (and roughly) determine software and OS of a server. Though their database is far from complete, “In late October 2006, Netcraft identified 100 million Web sites with domain names and content on them, of which 47 to 48 percent represented active sites.”