When you first view the Zimbra demo you may have a similar experience. Zimbra is, basically, a web based outlook/iCal/Thunderbird application in the same way that Writely is a web based version of Word.
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.
There are some core differences between how Zimbra and Writely approach their respective markets, however. Writely is a proprietary, hosted application (although they import and export in Word and other formats). Zimbra is an open source project, and is presented only in demo form at this point – if you want to run it you have to do so on your own servers.
Zimbra also integrates tagging of messages. It’s very impressive and quite beautiful.