Fascinating story in the latest New Scientist on measuring blog mood:
The software, called MoodViews, was created by Gilad Mishne and colleagues at Amsterdam University, The Netherlands. It tracks about 10 million blogs hosted by the US service LiveJournal.
The latest addition to Moodviews, a program called Moodsignals, tries to explain match these blogospheric mood swings to current events. It identifies emotional peaks by comparing recent label usage with records of previous use. When it finds a spike, the program picks out less commonly used words from relevant blog posts in an effort to identify the cause of the emotional change.