General Usability
Useit.com - Jakob Nielsen's Website Excellent website on usability
A List Apart - Website Design and Best Practices Journal
Web Style Guide - Excellent general usability site
Vincent Flanders - Examples of very bad websites
Colors
Colors on The Web - Guide to basic web color schemes
Color Blender - Mix and match color palettes
Typography
Web Style Guide Typography Section
Visual Impairments
Vischeck - Shows what websites look like to Color Blind website viewers
Jaws - Screen Reader software from Freedom Scientific
CSS