Found on WebResourcesDepot:

247 web usability guidelines to keep in mind while designing user interfaces.

The categories are:

• Home page usability
• Task orientation
• Navigation and information architecture
• Forms and data entry
• Trust and credibility
• Writing and content quality
• Page layout and visual design
• Search usability
• Help, feedback and error tolerance

Most of these guidelines are obvious, but there is a benefit to working off a defined list because it's next to impossible to keep in mind all 247 guidelines as you brainstorm designs and features and functions for your project.