TweetBeep is like Google Alerts but for Twitter. You don't even have to have a Twitter account in order to keep up with conversations that inlcude your chosen search term!
Simply enter "a keyword or website, and get emails when others tweet it!"
Just came across Twilert, which is another site that does exaxtly the same thing letting you "tap into the conversation and get a snapshot for what is being said in an easy and convenient way".
Technorati Tags: twitter, tweetbeep, twilert