CanIUse
Browser compatibility tables for HTML and CSS features etc
Code Magazine
"The leading independent resource for .NET developers"
CSS play
Excellent site with examples and explanation
Encosia
Articles on jQuery, AJAX etc
HTML to text
See what a web page looks like in a text-only browser
IRT.org
Tutorials and FAQs on web programming
iUI
Framework for developing mobile-friendly websites
jQuery
Very useful JavaScript library
QuirksMode
Browser compatibility tests for various standards
SproutCore
Open-source framework for building desktop-class web applications
Traffick
guide to portals and search engines
WebXACT
Detailed page validator covering accessibility, broken links etc.