An amusing infographic with some good thoughts on how to make solid decisions: http://shouldiworkforfree.com/clean.html (also available in the "f-bomb" or "sailor-mouth" version; I've supplied a link to the clean version).
A traditional blog post; if you can't answer "yes" to at least a few of the questions, then reconsider the offer you've received: http://www.cornonthejob.com/career-advice/will-work-for-practically-nothing/
News flash, a "duh!" wake-up call to employers: 20-somethings of the world are starting to resist the notion that they don't deserve pay for their work. Perhaps it's time to reconsider the internship: