IRC

Client-To-Client Protocol (CTCP) Documentation
Specifications for CTCP as well as DCC protocols which are now standard supplements of IRC.
http://www.irchelp.org/irchelp/rfc/ctcpspec.html

IRC+
An attempt to modernize the IRC protocol with backward compatibility in mind, making it easier to use. Includes an RFC draft and a FAQ section.
http://www.irc-plus.org/

Mirc.co.uk: DCC Resume
An extention to the standard DCC protocol which offers the option to simply restart a transfer at the point it was broken off and to proceed without loss of connection time.
http://www.mirc.co.uk/help/dccresum.txt

Most updated RFC1459
The original and authoritative RFC for IRC, updated by RFC 2810, 2811, 2812 and 2813. Written in 1993, it does not include details on DCC or CTCP, which came later.
http://www.irchelp.org/irchelp/rfc/

RFC 2810
Internet Relay Chat: Architecture. C. Kalt. April 2000. Updates RFC 1459.
http://tools.ietf.org/html/rfc2810

RFC 2811
Internet Relay Chat: Channel Management. C. Kalt. April 2000. Updates RFC 1459.
http://tools.ietf.org/html/rfc2811

RFC 2812
Internet Relay Chat: Client Protocol. C. Kalt. April 2000. Updates RFC 1459.
http://tools.ietf.org/html/rfc2812

RFC 2813
Internet Relay Chat: Server Protocol. C. Kalt. April 2000. Updates RFC 1459.
http://tools.ietf.org/html/rfc2813

Undernet FAQ Website
UnderNet's IRC-related FAQ
http://www.faqs.org/faqs/irc/undernet-faq/part1/index.html

Undernet Routing Committee
Resources for Undernet server operators.
http://routing-com.undernet.org/

(Yet Another) IRC Information Website
Contains links, explanations, guides, and many good documentations.
http://users.tkk.fi/~jlohikos/IRC.html