Here is a list of some of the very best resources (in no particular order) that I have found in the course of programming. I hope you'll find them useful!
Hg Init - Mercurial is a distributed version control system that is quite nice and powerful. The Hg Init tutorial is a great introduction to Mercurial, whether or not you've used a version control system before. Dive Into HTML5 - Dive Into HTML5 is a great free online book which describes some of the exciting features of the HTML5 specifications. Very informative and fun to read, it's a must for anyone who is interested in modern web development. Programming Perl, 3rd Edition - The Perl book (now in its fourth edition!) -- and my very first programming book that I picked up. (For all the details, visit my about page.) W3C Markup Validator - Make sure your web pages are valid! Firebug / Page Speed - Two of the very best web development extensions for Firefox in existence. Firebug lets you inspect and dynamically change pages as you edit, and Page Speed helps optimize your pages so they load quicker. SSL/TLS configuration for Apache - A nice blog post detailing how to configure Apache to use strong(er) SSL settings.