I mentor a couple of undergraduates. Beware of the fly-by-night start-up companies looking for (under)graduate students interns for free or inexpensive labor. Do thorough homework on any start-up you're looking to intern with and ask exactly what you'll be doing.
Especially be cautious of any internship program where the owner(s) will ask interns to sign a non-compete and/or non-disclosure agreement.
What city are you located in? Our company has posted internships in Boston recently on the sites below. If you're located in Boston, send me a message.
You should have no problem getting something. I can't think of a particular job board or the like, but startups would LOVE interns.
You could use the interships.com site that has recently been made famous by Charlie Sheen
Here is the Social Media intern job that he is hiring for
http://cs.internships.com/charlie-sheen-internship/?adly_id=1373351728 Apart from this job, I think there maybe a few applicants, there are many others worth taking a look at