Google Alerts is first and easy
Keyword searches on Google Blog Search - then subscribing to the keyword/phrase alerts: email alerts - RSS or feed to Google Reader.
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As Ed suggested monitorThis - Ed, that's funny, I stumbled across this one a few weeks ago.
Ed also suggested Andy Beals' Trackur is also a slick tool. I'm sure Andy has done quite well with it!
As Brian points out - as good as the free services are, they're not able to track a conversation as it threads out. Much like a review site. Think about it - if you're on the page of the company or service you are about to review, would you repeat the name of the company, most likely not. This is when you're not going to get the google alert or a keyword search to rss.
This is where this nifty little program come to good use, UpdatePatrol. You figure out how to get this going and you'll find a few other really cool features you can do with this - like know when all your local competition is making changes to their websites. 
You get all of the above plugged in and I think you're good on the capture side.
I'm going to start my own "Online Reputation Management Service!! 
