Blog search and blog review sites
Google, Blog Search, Technorati, BlogPulse (by Nielsen), Best of the Web, LeapFish, IceRocket, Trendpedia, BlogScope.net, Twingly, and blogcatalog.
TweetScan, Topsy, CrowdEye, TwitScoop, and DailyRT, etc.
Social-media sharing, recommendation & aggregation sites
Digg, Delicious,Reddit, Sphinn, and StumbleUpon.
BoardReader, BoardTracker and Omgili.
Social-media search and "realtime" search engines
SocialMention, OneRiot, Scoopler, Collecta, Wowd and LeapFish.
Subscription-based (paid) searching
One person also mentioned that they use Radian6 "so we can find exactly who mentions what topics, and more importantly access dashboards... to show who actually gets comments back, re-tweets, linked-to, etc."