Just posted Zimbra 4.5 RC1. It has several fixes and improvements from the beta feedback of customers, the community and our continued internal testing. A complete list of fixes and enhancements for this release can be found in bugzilla or with this query.
The big news in this release candidate is the first look at our new HTML lite client. Built on top the SOAP apis (like the AJAX client, Outlook, and Apple iSync) and some Zimbra JSP taglibs. This HTML lite client is targeted for users who don't have the network or compute resources for the AJAX client. Think modem/dial-up or those who haven't upgraded their computer in the last 5 years. You can access the HTML client from your upgraded 4.5 server at http://YOUR-SERVER/zimbra/h/.
Additionally we've added a new OS platform with Mandriva support.
There are still some known issues so any feedback would be greatly appreciated. Please report any bugs or requests via bugzilla.