Xobni -- 'inbox' spelled backward -- sees opportunity in helping users find and organize information contained within e-mail. Xobni's plug-in, currently in beta, keeps track of phone numbers, attachments and other types of pertinent data, displays it intuitively and classifies e-mail messages based on the user's relationship with the sender.

Xobni launched the public beta of its plug-in in May and says it has been downloaded more than 1.5 million times since then. The company has partnerships with the likes of LinkedIn, Facebook, Yahoo and Skype, all in an effort to bridge these forms of online interaction with the ubiquitous Outlook.

Last February at the Office Developers Conference, Bill Gates showered Xobni with praise during his opening keynote speech, calling it "the next generation of social networking."

Last April, Microsoft and Xobni were reportedly deep into talks for an acquisition in the neighborhood of $20 million, but Xobni rejected the deal because the company's founders didn't relish their potential role as a tiny speck in the Microsoft galaxy.

Other notable Xobni investors include Khosla Ventures, First Round Capital, Baseline Ventures and Atomico.