Here are the most popular/important/useful (in my opinion) RFCs. (RFCs (Requests For Comments) are files which define the online world. Great reading for those who can't get enough technical info and don't have anything else to do.)
RFC 791: IP, Internet Protocol
RFC 792: ICMP, Internet Control Message Protocol
RFC 793: TCP, Transmission Control Protocl
RFC 1118: The Hitchhikers Guide to the Internet
RFC 1180: A TCP/IP Tutorial
RFC 1661: PPP (Point-To-Point Protocol)
RFC 1855: Netiquette
RFC 1939: POP3
RFC 1983: Internet Users' Glossary