Astronomy Today
Offers information and articles on astronomy, space and related topics. Also featuring a friendly forum and a monthly sky guide.
http://www.astronomytoday.com/

The Astronomy Net
Contains articles, forums, clubs, vendors, manufacturers and other resources important to those interested in astronomy.
http://www.astronomy.net/

The Tenth Planet
Dedicated to chronicling the discovery and investigation of the tenth planet of our solar system. A repository of data about the newest planet for students, teachers, and amateur astronomists.
http://www.tenthplanet.info