Used to be when Roger Avary was the only screenwriter with a weblog. No more. Here are three other screenwriters' blogs that are well worth reading: JohnAugust.com: In addition to film credits such as Go, Big Fish, and the upcoming remake of Charlie & The Chocolate Factory, August answered script-related questions for years on imdb's message forums. His weblog consolidates all these resources into one, happy spot.
The Artful Writer, by co-authors Craig Mazin and Ted Elliott, strikes a very serious-sounding note by focusing on "information, theory, and debate for the professional television and film writer." One recent post, though, is an insightful and entertaining glossary of comedy writing terms [part I, part II], developed by the Zucker brothers and Jim Abraham to help "explain to each other why we're wrong."
Meanwhile, John Rogers (from Cosby to Catwoman has been assembling an excellent collection of comedy writing jargon, too, at his blog, Kung Fu Monkey.