The new beta from Zimbra™ looks promising. They make extensive use of AJAX technology -- the power behind Google Maps that makes server based applications behave more like desktop software.
Zimbra™ Collaboration Suite Open Source Project Beta Launch Zimbra is a community for building and maintaining next generation collaboration technology. Currently, this technology is available as a beta version. At Zimbra, our goal is to make e-mail, calendar, contacts and other communications technologies the best they can be. We believe that by opening the technology to the community we will insure that we can maximize innovation, scale and the ability to co-exist with existing messaging systems.
View the Zimbra™ Demo