I have added a new link on my front page: SFWA's Writer Beware blog, a comprehensive compendium of shady deals, borderline scams, and outright fraudulent agents and publishers looking to fleece new writers. There's also good information on how to spot a scam on your own. I'll add my own bit of advice here:
1. If the money is flowing from you at any point in the process, it's a scam (legitimate publishers assume all the financial risk publishing a book; legitimate agents are paid only when a client is paid). The only thing you should have to pay for during the publishing process is postage.
2. If an agent or publisher is soliciting submissions through advertisements, it's a scam (agents and publishers are always inundated with more submissions than they could ever accept). The only "advertisement" for publishers and agents that's reliably legitimate is the Writer's Market.