Hidden features of HTTP

Question

What hidden features of HTTP do you think are worth mentioning?

By hidden features I mean features that already are part of the standard but widely rath

Answer 1

ReST tries to push HTTP to its limits as an interface protocol.

It's not a hidden feature, but from looking at well-defined ReST APIs one can get quite a nice grip on how HTTP is meant to work and find wonderful examples of what can be achieved with simple combination of HTTP methods, status codes and headers to and fro.