Zimbra

Dreamhost does NOT support Zimbra at this time and cannot be installed by customers, as it require root access. See the instructions below on how to vote for DreamHost to add support for it.

About Zimbra
From the Zimbra home page.

Zimbra is open source server and client software for messaging and collaboration - email, group calendaring, contacts, and web document management and authoring. The Zimbra server is available for Linux, Mac OS X, appliances, and virtualization platforms. The Zimbra Web 2.0 Ajax client runs on Firefox, Safari, and IE, and features easy integration / mash-ups of web portals, business applications, and VoIP using web services.

Vote for Zimbra
If you are have a current webhosting contract with DreamHost and would like to see DreamHost officially offer Zimbra, you can cast your vote for it.