"I regularly use Twitter’s own search as well as TweetDeck and Kred (once I am logged in) to scan and search the hashtags and keywords.
But, Topsy is a game changer.
The free comprehensive index and social analysis tool, searches keywords, hashtags and @ signs from minutes ago or from across a span of years, retrieving your social needles from millions of haystacks of billions of tweets.
Topsy is a way to instantly discover breaking news and just released press-releases and track current conversations and just posted media."
Via Howard Rheingold, luiy, Pierre Levy