Free open access to journal articles seems like a great thing -- for readers. For the authors it often means a heavy payment to publish if your article is accepted. Apparently now there are a number of publishers that are not up front about the fee, often inviting potential authors without disclosure. For more information on this topic, take a look at the blog Scholarly Open Access by academic librarian Jeffrey Beall, who follows and investigates the world of scholarly open access publishing. His site includes lists of questionable journals and publishers.
1-29-2017 update: Beall's List has been taken down and currently its absence is unexplained.