There are two main versions of SOCKS -- V4 and V5. V5 adds an authentication mechanism for additional security. There are many freeware implementations of both versions. One of the most common V5 implementations is SOCKS5, developed by NEC.
SOCKS was recently accepted as an IETF standard and is documented in RFC 1928, 1929 and 1961.
Introduction to SOCKS Provides a good overview of SOCKS, including some helpful diagrams.
SOCKS FAQ Detailed and up-to-date FAQ on SOCKS. Maintained by several engineers at NEC.
SOCKS home page Home page for the unofficial SOCKS web site hosted by NEC.
SOCKS Resources Index of Web pages and sites related to SOCKS. The page is maintained by Aventail Corporation, a leading provider of VPN solutions.
SOCKS5 FAQ Answers questions about NEC's implementation of the SOCK V5 protocol.