Bit.ly is the biggest URL shortening service thanks to the popularity of Twitter.  Given that websites and files are masked by Bit.ly, spammers and virus artists are taking advantage of the situation.  A normal message may be followed by a Bit.ly URL that links to a malicious website.  In a blog post from earlier today, Bit.ly announced that they are partnering with VeriSign, Websense, and Sophos to scan through links and identify which of them are spam are viruses.  Bit.ly has about $2 million in funding and became the Twitter default URL shortening service in May 2009.  Bit.ly is based in New York City.