Nice summary of good listening aggregators in social media here by Chris Brogan. Nothing you wouldn't know if you read this blog regularly, except this:
Here’s an off-the-beaten path one. Go to SEO Pro and use their free link checker. (Note: it’s a bit slow to crawl for technical reasons, so don’t get worried if it takes a while to respond to your query). This tool checks who’s linking to your URLs, what the link text is (what’s in blue on the web page that people might click to get to you), and all kinds of stats that matter to search engine optimization experts, but might not matter to you. Why? Because it’s important to know what people are saying about you with their linking efforts.
He also talks about professional buzz measurement systems, but as we build 'em too we'd far rather you got one from us