Unethical publishers, sometimes called predatory publishers, are the Nigerian princes of the open access world. There is a growing industry made of "publishers" that operate in a couple of ways. Some contact new authors (particularly graduate/undergraduate student authors) and "offer" to publish their work in exchange for the copyright. They typically post the work on amazon or other sites and print a copy on demand for anyone who orders it (usually almost no one). Others create a fake journal which is not peer-reviewed or edited, and they charge authors for publishing in said journal.
If you receive a "too good to be true" communication about your scholarship, please be skeptical. Librarians are happy to help investigate the legitimacy of such communications.
See I Sold My Undergraduate Thesis to a Print Content Farm (Slate, 3/23/14), especially if you are contacted by "Lambert Academic Publishing."
Jeffrey Beall, a librarian at UC-Denver, keeps a list of potential, possible, or probable predatory scholarly open access journals.
Also follow Beall's Scholarly Open Access blog for current developments on the issue.
Finally, this 4/7/13 NYT piece provides great background on this topic: Scientific Articles Accepted (Personal Checks, Too)