RateMyProfessor.com lets students do just that. These are my favorite comments from their list of funniest responses...

• You can't cheat in her class because no one knows the answers.
• Three of my friends got A's in his class and my friends are dumb.
• Emotional scarring may fade away, but that big fat F on your transcript won't.
• BORING! But I learned there are 137 tiles on the ceiling.
• Teaches well, invites questions and then insults you for 20 minutes.
• I learned how to hate a language I already know.
• Bring a pillow.
• Your pillow will need a pillow.
• If I was tested on her family, I would have gotten an A.
• She hates you already.

Full list available at: http://www.ratemyprofessors.com/Funniest.jsp