One of the few sure things on the web is that if something isn't easy to use, it won't get much use. Just as E-commerce companies need to be concerned with potential customers being able to use the company site to find what they want, faculty who construct online educational resources should consider usability issues to make sure that the design doesn't pose yet another obstacle to learning. There is a good bit of information available on Usability, and while most of it is aimed at the commercial market, it is usually applicable to a generic web site and so to online educational resources.
The book Don't Make Me Think; A Common Sense Approach to Web Usability, written by Usability Consultant Steve Krug (Krug, 2000 ), provides many helpful suggestions illustrated by before and after examples. The book highlights the importance of things like simple, consistent site navigation, clear organization of ideas, and usability testing during the entire development process. It also attempts to dispel many misconceptions that people have about web design. The author also maintains a companion website, Advanced Common Sense (more info) , where you can get additional information.
The Usability 101 (more info) issue of Jakob Nielsen's Alertbox column on useit.com gives an in-depth introduction to usability issues: what is it, why is it important, how can you improve it on your sites, etc. There are also links to his other writings dealing with Usability issues. There is a lot information here, and although the author uses a fair amount of web jargon, he generally does a good job of explaining it or linking to another area where it is explained.