Is your non-profit site accessible? Check and see at http://www.cast.org/bobby/
For information on the subject go to Non-Profit Site Analyzer
Or you can review the Disability Etiquette Handbook

Final notes from Jayne Craven's Tips Pages.

Idocs HTML Code Tutorial

Websolutions' Tips for Website Maintenance

Proboost.com
Free Meta-tag check

Anybrowser.com
Check out how your site looks as viewed from different browsers.

Netmechanic
Give Your Site a Tune Up! Find problems before your visitors do!