I frequent quite a few blogs, many of them posted by professional screenwriters, literary agents, and editors. While some are listed in my blogroll (left), I thought I'd post them and some others, along with commentary on why I read them regularly.
John August - He's written many successful films, including Big Fish, Charlie's Angels, Corpse Bride and Charlie and the Chocolate Factory. He also wrote one of my favorite dark comedies, Go. His blog is filled with useful, and very practical, information for screenwriters. Lately he's been documenting his experiences as the director of The Nines, a film he also wrote. At the time of this writing, The Nines is in limited release.
Ken Levine - He was a head writer for M*A*S*H (one of my favorite shows), and has written for The Simpsons, Cheers, Frasier, and many more shows. While he mostly posts reviews (including a comprehensive recap of American Idol, which appears to be his favorite show), favorite clips of famous comedy shows, and personal anecdotes, he also occasionally shares script samples and great tips for screenwriters, particularly TV writers.
Alex Epstein (Complications Ensue) - A Canadian screenwriter and former development executive. He also wrote Crafty Screenwriting (which I read and found very informative) and Crafty TV Writing. He posts regularly, and often answers reader questions.
Jane Espenson (Jane In Progress) - A TV screenwriter/producer with credits like Buffy (she worked with Joss, how cool is that?), Battlestar Galactica, Tru Calling, and Eureka. She answers reader questions often, and offers a lot of great advice for aspiring TV writers.
Pub Rants - One of the better blogging agents (the best, Miss Snark, is gone; more on that in a sec.) She answers publishing, querying, and writing questions, and offers insider advice on the publishing industry.
Miss Snark, the literary agent - Though she retired, her informative, witty, blunt, and, well... snarky blog remains available. This is a must read for any aspiring writer.
Evil Editor, why you don't get published - A humorous look at the slush pile. With staples like New Beginning, which allows blog readers (minions) to finish someone's opening page, and Face Lift, an amusing critique of submitted queries, this blog illustrates why most writing is unpublishable. But you definitely learn a lot as you laugh, and cry.
Flogging the quill - Home of the Flogometer. Ray Rhamey takes a more serious look at how hard it is to get published. He evaluates the first page of your novel, and explains in great detail why he would or wouldn't turn the page. His editing comments are well worth the exercise.
Just Plain Funny
101 Reasons to Stop Writing - You might as well give up right now; it's just not worth it. That's the message of this site. This blog is a humorous look at how bad most writing is, and why you wouldn't even want to be a writer in the first place. Not everyone finds it funny, as you will see in the comments, but the posts are often hilarious--and sometimes informative. And don't forget to check out the worst cliche' poll.