Zarafa Collaboration Platform is an open source Exchange replacement solution built on ClearOS Linux.
Yet another viable open source replacement for Microsoft Exchange, Zarafa is completely free with unlimited browser based access or with up to three Outlook clients but without calendar sharing. If you require more Outlook connections then the commercial editions have a purchase cost and annual subscription.
The software is easier to install and configure and updates are supported with the product very closely integrated with the operating system. This is a very good Exchange alternative and while it is open source, lack of calendar sharing on a collaboration just doesnt cut it and therefore this is essentially a commercial product and won't suit every budget.