I've noticed some highly questionable unpaid internships listed on UMBCworks lately. Here are the requirements that must be met if the internship is legal:

1. The internship, even though it includes actual operation of the facilities of the employer, is similar to training which would be given in an educational environment;
2. The internship experience is for the benefit of the intern;
3. The intern does not displace regular employees, but works under close supervision of existing staff;
4. The employer that provides the training derives no immediate advantage from the activities of the intern; and on occasion its operations may actually be impeded;
5. The intern is not necessarily entitled to a job at the conclusion of the internship; and
6. The employer and the intern understand that the intern is not entitled to wages for the time spent in the internship.

Source (with more information): http://www.dol.gov/whd/regs/compliance/whdfs71.htm

More resources:

Propublica's breakdown on when unpaid internships are legal: http://www.propublica.org/article/when-interns-should-be-paid-explained

Legality of unpaid internships at not-for-profit organizations: http://www.fracturedatlas.org/site/blog/2013/08/14/avoiding-the-black-swan-part-ii/#Buttons