RFC Editor
Collates Request For Comments documents. Explains the RFC process, and includes a searchable database of RFCs.

IETF Internet Drafts
List of documents organized by alphabetized subjects.

Active IETF Working Groups
A directory of the Internet Engineering Task Force groups and their web pages.

Internet protocols
General comments on protocol design, and good reference material on a few protocols (especially mail-related; also ftp, time, and misc) by Dan Bernstein.

Port Numbers and Services Database
Well known port numbers for TCP and UDP, from Internet Assigned Numbers Authority (IANA).

RFC Sourcebook
Description of protocols, file formats, glossary of terms, RFCs.

Protocols.com
Features an overview of various networking and mobile communications protocols, white papers, and links of interest.

XNS.org
Independent organization managing the eXtensible Name Service, an open protocol and open-source platform for universal addressing, automated data exchange, and privacy control. Includes an overview, charter, history, and specifications.

Internet Encyclopedia
Overview of Internet protocols.

The Kermit Project
A protocol that transfers text and binary files without errors between diverse types of computers over potentially hostile communication links, and a suite of communications software programs; Columbia University.

Free Protocols Foundation
Independent public forum supporting patent-free protocols. Includes policies, papers, a mission statement, and a list of free protocols.

The Internet Report (IETF Weekly Updates)
A table of new Internet Drafts and RFC's that come out each week, with synopses. Archived summaries go back to June 1999.