Adobe PostScript SDK Archive - A compilation of documents and sample code previously distributed on CD as the Print Technologies software development kit. - http://partners.adobe.com/public/developer/ps/sdk/index_archive.html
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. - http://atrey.karlin.mff.cuni.cz/~milanek/PostScript/Reference/
PostScript Books - Lists a couple of Adobe and third-party publications. - http://www.cs.wisc.edu/~ghost/doc/books.htm
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. - http://partners.adobe.com/public/developer/ps/index_specs.html
Adobe PostScript - The official site of PostScript, with resources, information, and links. - http://www.adobe.com/products/postscript/
Wikipedia: PostScript - Encylopedia article about the language and document format. - http://en.wikipedia.org/wiki/PostScript
The PostScript Area - Overview and history of language, explanation on how PostScript RIPs work, troubleshooting database. - http://www.prepressure.com/ps/psoverview.htm
Examples from the Blue book - Excerpts covering basic graphics, printing text, applications, and modifying and creating fonts. - http://www.science.uva.nl/~robbert/ps/bluebook/