Every year, Writer's Digest rounds up the websites it considers best for writers, in a wide variety of genres and subject areas. So if you don't subscribe to WD, the May/June 2011 is definitely an issue you want to pick up! I won't list them all (there are over 100), but here are some that I visit on a (semi) regular basis and can vouch for their appeal/helpfulness:
Agent Query - find any agent here, listed by subject area/interest
BookEnds - agents Jessica Faust and Kim Lionetti run this blog
Pub Rants - agent Kristin Nelson has run this blog for years. Hugely helpful in terms of what agents are looking for, and she has a few series of posts regarding query letters, pubbing, etc.
Media Bistro - offers tons of resources & industry news for freelance writers
Preditors & Editors - lists agents and publishing houses along with questionable news, those who are fraudulent/not to be trusted, etc. A must-check before writers query ANYONE
Nathan Bransford - used to be an agent, now a writer, has friendly, easy-to-read posts on the inside workings of the publishing world.
WOW - Women on Writing - I freelanced for them a few times. Website devoted to supporting women's writing endeavors, really it just provides some great writing tips, industry news, etc, that anyone could find useful.
Absolute Write Forums - I check in there at least once a day. You can find information on just about any area of writing you're seeking. Forums for newbies to multi-published authors and anyone inbetween, with just about every genre and sub-genre covered. Awesome!
Happy writing/reading/researching/web surfing!