I ran across this site once and let Twitrratr slip by. While it isn’t always exactly accurate, it gave a good indicator of how a watched keyword is being presented (as in a positive or negative manner).
Discover what people are really saying on Twitter. With Twitrratr you can distinguish negative from positive tweets surrounding a brand, product, person or topic.
There is no sign-up or login, just enter a term. It would be nice to get this as an air app or widget eventually.