The Government Domain column in LLRX.com "The Cream of the Federal Web Site Crop"  by Peggy Garvin discusses the best of them and why usability is so important to having a good website. She ends the column with a site recommendation "that may restore your faith in government websites."
Usability.gov  from the Department of Health and Human Services is a "resource for designing usable, useful, and accessible web sites and user interfaces." The site began as a project of the National Cancer Institute to find evidence-based usability guidelines so that they could make cancer information easily available to the public. The product, Usability.gov, is available to the public as well as federal web developers. Several of the World Usability Day panelists remarked that a plus to working in government is that they can "steal," or adapt, the work of their federal colleagues for their own projects, and agencies actively share their solutions within the federal community.