Adobe PostScript
The official site of PostScript has, among others, the PostScript Language Reference. (Adobe)

Richard's PS Junk Page
PostScript news, downloads, documentation.

PostScript Introduction
An overview of the PostScript page description language, where it came from, and the problem that it solves.

PostScript and GhostScript Resources
Links to general information, utilities, and software.

A First Guide to PostScript
A tutorial of the basics of the PostScript page description language. Includes examples.

Dylan's PostScript Page
A few handcrafted PostScript programs illustrating the data format's structure and basic features.

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.

EPS in Ten Easy Stages
Brief introduction to Encapsulated PostScript.

PostScript Books
Lists a couple of Adobe and third-party publications.

PostScript Programming Tutorial
Step-by-step guide to writing PostScript programs by DevCentral Learning Center. Site registration (free) is required to view this guide.