The wonderful, brilliant and incredibly talented TV writer Jane Espenson’s website is full of great wisdom and advice about writing for television. My favorite thing is how she concludes all her blog posts with what she had for lunch.
John August is an A-list screenwriter, app developer, prolific blogger (his site, which he created in 2003, has over 1,500 posts), and all around Renaissance man. He also generously posts versions of his treatments, concept docs, outlines and scripts.
Wordplay is a site by Terry Rossio, who co-wrote ALLADIN, SHREK, the PIRATES OF THE CARIBBEAN films, and many others. This site also has lots of great screenwriting info, forums, columns by guest writers, and great advice written by Rossio himself.