iCal Server and Mail Services. There's also Wiki Server (collaboration) and iChat Server (a presenter's wet dream in addition to managing IMs and chats). All included, no client licenses.
Migration from Exchange to iCal looks to be a “tear-it-out-and-start-all-over” arrangement. It also looks like iCal suffers from the same problems with large scale free/busy searches that Exchange did until the current release - forcing the client to publish the free/busy information, and making the searching of calendars a client-side operation. I just got out from under that arrangement and I’m in no hurry to go back.