Pub Rants - I put Kristin's first not only because I have a personal stake there, but also because she gives a ton of great advice and insight into the world of writing, agenting, and publishing.
Miss Snark - Is there anyone who doesn't know her? Questions about all aspects of the publishing industry answered in her very own snarkalicious style (in other words, lots of sarcasm), plus periodic critique opportunities. I found Kristin's blog through hers, for which I am eternally grateful.
Lit Agent X - An up and coming agent who represents urban fantasy and YA, who shares the best and the worst of the query pitches that come her way, along with posts lots of other topics.
Agent and Publisher Research
Publishers Marketplace - The most comprehensive (that I know of) database of publishing and agent information. But mostly I like it for the deals section. You can find out what's selling and who's selling what (for about how much) in just a few seconds. Definitely worth the subscription price.
Agent Query - If I've checked up on an agent at Publishers Marketplace, it's probably because I found them here. Searching by genre helped me find most of the agents I eventually queried for both Bad Luck Boy and In Memory Of.
Submitting to the Black Hole - The place to check out response times for speculative fiction publishers and magazines. I'm not visiting there very often now that my short story submissions have dropped off, but I've gotten a lot of use out of it in the last five years.
Children's Writer's & Illustrator's Chat Board - My new favourite hang-out. All the members (some published, some aspiring, writing everything from picture books to YA novels) are friendly and helpful, and they've got the best agent response time section I've found.
The SFF Online Writing Workshop - I haven't been a member for a couple of years, because I found it didn't work so well for getting novel critiques, but I got lots of amazing feedback from other members when I was submitting short stories, and many of my current LJ friends I found through the OWW.
And of course there's my favourite LJ communities, which you can see in my profile.
If I think of any more, I'll add 'em. Feel free to share your own favourite writerly sites.