What are your favourite RFCs?

It seems there is an RFC for everything, from RFC2-119 (which explains the difference between MUST, SHALL and MAY), to RFC-1178 (Choosing a name for your computer), to the more serious RFC-3339: Timestamps. There is a significant amount of thought put into these RFCs, which often goes beyond the mere technicalities of the internet.

What other RFCs do you find fascinating, especially those that are non-technical?