Well, You'll see one coming shortly from Net Integration Technologies - we're currently in Beta for a product that allows Outlook, Webmail and Evolution to all talk to each other, exchange Calendaring information (including delegating user rights, etc.), assign Todo's, integrate contact lists, and pretty much all of the other commonly used features of Exchange.

(We're the same people who brought you WvDial, and a bunch of other stuff - check out open.nit.ca for more of the projects that we have done.