Great list of different educational networks in many different formats, including diigo, Linked In, Facebook, nings, wikis, and more. Some are by discipline
Kathy Schrock has an extensive guide to the best websites. This particular page summarizes the sites she has recommended in the past. (It started more as daily, and is more like weekly now.) Her entire site is very organized, and has something for every discipline.
Interesting list-- includes educators from many different walks, including some in specific disciplines. Click on a name to learn how to connect with them.
Instructional guides with model that include authentic performance of PBL assessments with accompanying rubrics. Separated by discipline, including the arts. Rec. by Megan Utter