TFTP

Manual page for TFTPD(8C)
Describes the TFTPD server.
http://www.cs.rit.edu/~hpb/Man/_Man_SunOS_4.1.3_html/html8/tftpd.8c.html

pumpKIN
Open source implementation for Windows of a TFTP server and client. Includes a brief presentation, a technical description, help files and a screenshot.
http://kin.klever.net/pumpkin/

RFC 1785
TFTP Option Negotiation Analysis. G. Malkin, A. Harkin. March 1995.
http://tools.ietf.org/html/rfc1785

RFC 2090
TFTP Multicast Option. A. Emberson. February 1997.
http://tools.ietf.org/html/rfc2090

Wikipedia: Trivial File Transfer Protocol
Describes the term 'TFTP', explains the protocol and the details of a session.
http://en.wikipedia.org/wiki/Trivial_File_Transfer_Protocol