OneSangha

http://www.berzinarchives.com/web/en/index.html/ is one of my favorites with lots of teachings and historical info too.

http://www.tibetanlineages.org/ is a new (to me) site with biographies of the masters of all the Vajrayana lineages and Bon masters as well.

http://www.himalayanart.org/ is an incredible repository of artworks of all kinds from Tibet and surrounding countries, great for Yidam pictures. Try browsing through the sculpture or textiles to see some images you might not normally find.

http://www.lotsawahouse.org/ has many translations of traditional texts. this page of the most commonly cited quotations in Tibetan Dharma texts is just amazing http://www.lotsawaschool.org/quotations.html/

http://www.simplybeing.co.uk/simply/Transcripts_Dzogchen.html/ has a number of teachings from James Low who is one of the best western born Dzogchen teachers imho.

http://blazing-splendor.blogspot.com/ mentioned this one in a post here on another forum about my favorite Dharma blogs, but I will cite it again because it’s my favorite Vajrayana blog.