twitrratr looks like an interesting attempt to provide some sentiment analysis to Twitter. Do a search for a keyword and twitrratr displays the tweets matching that term, with entries organized in 3 columns: negative, neutral, positive. Some algorithm works in the background to determine the sentiment.
The major issue I have with sentiment analysis is that it is not trivial stuff. From what I've seen of vendors who provide this service to corporations doing analysis of product mentions on the web, the effectiveness of this type of analysis is dependent on the algorithms that crunch text, the amount of text, and language used. Because Tweets are short I wonder how effective they can be. The upside to the constraints of a 140 character textarea is that often people end up being more concise and better communicate what's on their mind. Other times, though, it can be utterly meaningless.