If you write Science Fiction, Fantasy, or the otherwise undefinable Speculative Fiction, check out the Science Fiction & Fantasy Writers of America's Writer Beware site. They publish a list of Alerts for Writers, case-studies of Scams in Action, a thumbs-down list of publishers and agents not to query, and more. Nobody wants to focus on the negative, but it's good to be aware. They also have a blog for up-to-the-minute warnings and other helpful writing thoughts and information.
I came across the link to Writer Beware in Ralan's Webstravaganza market newsletter. If you're interested in Writer Beware you'll probably also be interested in Ralan's market listings - Check out his site and sign up. It's spam free.