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

    CSS Beauty