Some suggestions in these Quora threads:

http://www.quora.com/Is-there-a-feedback-plugin-that-I-can-u...

http://www.quora.com/What-is-the-best-website-feedback-widge...

Or, a couple of jQuery examples (there are probably others):

http://plugins.jquery.com/project/imfeedback

http://wpaoli.building58.com/2009/08/jquery-animated-feedbac...