Writing Forums:Absolute Write. I'm on several forums, but AW is the most active and has the most subforums for finding others with similar interests. From general writing advice to genre specific questions, to self-publishing and even blogging, there's a ton of great people on there to help. And it has a real community feel where everyone looks after each other, people argue and make up, and there's that weird guy in the corner everyone avoids. You can just lurk and learn a lot, but getting involved speeds the learning curve.
Duotrope: Great resource for finding markets to sell short stories.
Querytracker. There are several sites to find information on Agents, but I like their search features and useful links.
Blogs are a varied breed, some are useful, some are entertaining. What's good really depends on what you're looking for.
Maybe Genius. Stephanie Sinkhorn, YA writer. Very knowledgeable in the field of YA with entertaining and thoughtful posts on writing and the industry.
Shannon Knight. Good book reviews if you like that kind of thing (I don't). What I like are her posts on myths in literature - rather scholarly but that's what I find useful and interesting.
Rachelle Gardner. An agent blog. Good industry information for aspiring writers.
Confessions. Agent Suzie Townsend. Her weekly Query Roundup gives a good insight into what people are submitting and, more importantly, what catches an agents eye.
I'm even newer to Twitter (@blairbburke), maybe a month in and still figuring out how useful it really is. It can be a huge time suck if you let it and it's hard to separate the signal from the noise. So far I think the best use is to follow agents, hopefully ones who represent your genre. You can get some good insight into their world and useful tips on the querying process.
Sarah LaPolla (@sarahlapolla) tends to have a good amount of useful info without too much filler about cats.
There's probably a lot of additional useful sites that I haven't found yet, but these are a good starting place. Anyone else have any favorites that they'd like to add?