FreeBSD: Sockets: FreeBSD Developers' Handbook chapter about using sockets
RFC 2292: Advanced Sockets API for IPv6: Defines some the advanced features of the sockets API that are required for applications to take advantage of additional features of IPv6.
sockaddr.com: Programming Resources for WinSock 1.1 and 2 including sample programs, specifications, and links. (Lewis Napper)
TCP/IP Raw Sockets: MSDN documentation about Raw Socket specification. Raw sockets can be used to create your own protocols.
Windows Sockets 2: Developer documentation of Winsock for all Windows platforms.
Windows Sockets Programming: Links to get started with WinSock programming. (Ed Snible)
WinSock Development Information: Sample programs, technical articles, links, and an extensive WinSock error reference. (Bob Quinn)