I have a friend who just emailed a ton of professors and asked them if they had any room in their labs.
This. Find an area you like, and send out a bunch of emails to researchers doing work in this area. The body of email can be like a cover letter, and make sure to attach you resume.
Hi Dr. X, My name is anna726, I'm an undergrad bio/chem/psych/whatever major at State U. My interests include A, B, and C, and I'm looking for a summer research position. I've noticed that your lab does research on (A, B, C, or some combo thereof), and this is an area I'm very interested in. Are you planning on hiring any undergraduate RAs for the summer of 2011?
Etc. Fill it out more, and be courteous. Not everyone will get back to you (expect ~50% response rate, depending on who you email.) Out of those, not everyone will have enough grant money or positions to hire you for the summer. Still, if you email enough people, you're bound to turn up something good.
I took this route last summer. Emailed 20 or so neuroscience/psychiatry researchers. Heard back from 10, got asked for to come in for an interview at 4 or 5. Worked out great for me.
There are also some formal summer research programs, but I'll pass those off to people who know better. I believe those tend to pay a bit better, but are also much more competitive. Hope that helped!