You can see what is coming....sentiment trackers are largely expensive and disappointing in my opinion. They do not pick up every online brand mention and are not great at analysing the complex language issues of saying whether the articles have a positive or negative sentiment. Computers aren't good at understanding sarcasm and the complexities of language and slang.
But Google has now bought Post Rank. It appears they want the development team rather than the product. This is their 4th such acquisition in recent times. Surely a low cost but effective sentiment tracker will be on its way soon. Here is a great article which outlines more about the Post Rank deal