As you probably know Twitter is hugely popular and countless numbers of tweets (micro blog posts) contain links to other sites.
One problem with embedding URLs at Twitter is that at some point Twitter will convert URLs using URL shortening services, such as http://bit.ly
When URLs are altered by such services a lot of commonly used URLs scanners can't detect potentially malicious sites. So it makes sense that someone would create a tool to help protect users from malicious URLs embedded into tweets.
Finjan now offers SecureTweets for Firefox and Internet Explorer. It's a browser add-on that offers a "look ahead" feature that can peek into the site behind a shortened URL to see if it might be dangerous before you decide to click on it.