Mary Fran's Google Sites Tutorial is an excellent resource for anyone using Google Sites.
She has done an excellent job of breaking down different aspects and functions of Google Sites into an easy to read and follow format.
The site (created on Google sites of course) includes getting started, the basics, editing, inserting gadgets, using layouts and templates, and much more. She has video tutorials, links, and examples of sites and also has a section specifically for educators.
If you use Google Sites or want to create your own web site using Google Sites, I highly recommend this site. It is also a great resource for students who are using Google Sites.