TweetFeel was sent to us yesterday to poke around with. It appears Mashable listed it and overloaded their site so be patient.
You simply enter a search term (I chose Mixero as a test) and it scans tweets looking for terms around it that would show how a person feels. It then gives a percentage rating between smiley faces and frowns.
A simple approach to try and gauge how the public is reacting towards a product, person, event or whatever you dream up. The results fill the screen as it counts so you can see the tweets themselves. Once they build in the ability to authenticate and retweet or reply to specific ones that will be great.