Adobe PostScript: The official site of PostScript, with resources, information, and links.
Adobe PostScript Language Specifications: Official specification of the core data format/language and of several of its extensions, specification errata and supplements. Hosted by the Adobe Solutions Network.
Adobe PostScript SDK Archive: A compilation of documents and sample code previously distributed on CD as the Print Technologies software development kit.
Don Lancaster's PostScript Library: Essays and examples on PostScript programming.
Examples from the Blue book: Excerpts covering basic graphics, printing text, applications, and modifying and creating fonts.
Luc Devroye's PostScript Page: Links to various Resources.
PostScript and GhostScript Resources: Links to general information, utilities, and software.
PostScript Books: Lists a couple of Adobe and third-party publications.
PostScript Language Reference: Complete list of PostScript operators, operands, and symbols. Also features a PostScript troubleshooting section, information on document structuring conventions, the OpenPress Interface (OPI) specification, and a list of books.
PostScript Language Referenz: Extensive, third edition.
PostScript Manual: David Maxwell's introduction to programming in PostScript.
PostScript Processing Speed Test: Benchmark for PostScript printers.
Richard's PS Junk Page: PostScript news, downloads, documentation.
The PostScript Area: Overview and history of language, explanation on how PostScript RIPs work, troubleshooting database.
Thinking In PostScript: Glenn Reid's book, generously made available in PDF format.
Wikipedia: PostScript: Encylopedia article about the language and document format.