Last week, Dan Farber provided a look at the next step in the evolution of the Zimbra Office which features two "notebook" apps - a document processor and a spreadhseet and a new approach to sharing data between web-based applications called ALE (AJAX Linking Embedding). More than anything Google or the all-software-should-be-free contingent is doing, this is what Microsoft needs to fear - a disruptive player coming out of nowhere, delivering solid usability in a well-designed interface that takes on a cash cow (in this case Exchange Server).
Exchange is an expensive proposition. Zimbra is not - an open source version is available as are a Network Edition that adds value and support for a fee and third-party hosted versions. This project has ramped up quickly, has solid financial backing, and seems to have a long view strategy in place that is being executed with deliberate intent. That should raise a few eyebrows in Redmond.
UPDATE: Richard MacManus has posted an in-depth walkthrough of Zimbra that shows off some of the very nice features this alternative provides.