SED, the Stream Editor: Eric Pement's Cornerstone Mag site hosts an extensive FAQ, download sites for most operating systems, large list of handy one-liners, table of version differences, exhaustive links.
SED: The Open Group "Single UNIX Specification" man page.
SED & AWK: The most popular book on the stream editor, from O'Reilly.
SED FAQ (He Sed She Sed): Dreamwvr.com's alternate frequently asked questions list is arranged in increasing order of difficulty, and can be read as a tutorial.
SED Tutorials: Some items selected by Yiorgos Adamopoulos from the Seders grab bag and mailing list.
