Zimbra

Zimbra is an open source server and client technology for next-generation enterprise messaging and collaboration. Zimbra delivers innovation for both the administrator and the end-user as well as compatibility with existing infrastructure and applications (both open source and proprietary). Zimbra is open source in order to leverage the collective talent of the greater community, and to ensure investment protection for Zimbra partners and end-users.

Some press about the company here from USA Today:

A company called Zimbra generated tons of buzz at the conference, and it's based on AJAX. Zimbra uses the Web to offer high-end corporate-style e-mail — doing for Microsoft Outlook users what Gmail did for consumer e-mail users.

Zimbra's service blurs the definitions among individual PCs, corporate servers and the Web. Which makes Zimbra a Web 2.0 company.

Plus, by offering corporate e-mail, the company is taking on Microsoft in a way no one dared a few years ago. Which also makes Zimbra a Web 2.0 company.