W3C Markup Validation Service, a free service that checks Web documents in formats like HTML and XHTML for conformance to W3C Recommendations and other standards.
“For people who make websites” A List Apart Magazine explores the design, development, and meaning of web content, with a special focus on techniques and benefits of designing with web standards.
a system that helps web site designers in the early stages of design. DENIM supports sketching input, allows design at different refinement levels, and unifies the levels through zooming
Build a Feed You Can Cut 'n Paste, will help you format a feed's display with the information you want to use on your web site. All you need to enter is the URL for the RSS source.
a bible of transitioning and progressive enhancement back when nobody was certain about the direction of HTML working groups and browser support. By Mark Pilgrim
integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
makes screenshots of web design in different browsers. It is a free open-source online service created by Johann C. Rocholl. You submit a web address, it is added to the job queue. Distributed computers will open the site and make screenshots available