Outlook-like apps for Linux?

Troy Jacobson writes in with this dilemma: "I'm facing what could be the start of a migration to NT. One major app that has been desired is Exchange, so that people could use some advanced features of Outlook (e.g., the shared calendar and contact management). Do any alternatives to Exchange exist that run on Linux?" Now I don't know if any single solution exists, but isn't there a suite of apps for Linux one can run to achieve something close to Outlook's functionality?