A company which performs surveys of the InterNet and the WorldWideWeb.

What Netcraft does is not particularly difficult, but they've been doing it every month since 1995, so (a) they're good at it (b) they have data over time which is very useful and (c) they're not obviously biased towards a particular vendor.

The best known of their surveys involve determining what OperatingSystem and WebServer a host is running, but they also survey UpTime and delays to a lesser extent.