BackTweets is a twitter time machine which enables you to search through a tweet history for tweets that link back to your site.
As a free web app, BackTweets lets you search through a tweet archive for URLs sent via twitter. URLs are found regardless of the form they were tweeted in and include: full URL links, shortened URLs and URLs without the WWW prefix. BackTweets is a great tool for tracking your social media pulse- see how many people are talking about you, who is talking about you, and what they are saying.
You can find more information about the current version of backtweets on the pages linked-to above.