Preditors and Editors is a great site for aspiring writers. The whole thing is devoted to investigating publishers, agents, etc. and reporting on those that are reputable versus those that are scammers (and there are a lot of scammers out there).
Some examples - agents who charge reading fees, PublishAmerica, agents who never sell anything, publishers who make most of their money off of books they sell back to the author.
I have relied on Preditors and Editors several times in the past, especially when reading about a publisher or agent that seemed too good to be true. The site doesn't cost anything and it has great information. Enough so that some publishers have cleaned up their act and even gone to great lengths to work with P&E to ensure the site they are legit.
I thought about this recently when I saw a writer on the fantasy convention circuit who had published through Publish America. The name rang a bell so I went to their site and read about them and it sounded too good to be true, so I checked Preditors and Editors. Yikes. Avoid PublishAmerica. But don't avoid Preditors and Editors. In fact, visit often.