I've often called blogs early warning systems for popular trends. Many bloggers pride themselves on keeping ahead of the curve and writing about topics or events before the mainstream media catches on. The blogosphere also makes it easy for dialogs and conversations to arise, and with the proper tools you can really get a sense of what's hot and what's not simply by tracking this blog activity.
In today's SearchDay article, Tracking Trends via the Blogosphere, I review BlogPulse, a blog search engine from Intelliseek. BlogPulse has a cool comparison search tool that lets you see the ebbs and flows in popularity of a search term over time, and also lets you map one or more terms against others to get a sense of the relative popularity of each subject.