Are there any good open-source Exchange 2003 clients for linux?
That can handle all the Exchange features like Calendars, etc.
If you want full Exchange features then as Roy mention, the only real option with linux is Evolution.
There have been some attempts to create libraries for use with Exchange in the OpenChange project, so hopefully something will come of that.
Other than that you would be limited to either using OWA (which is not great on Exchange 2003) or IMAP/POP connectivity.
There is an interesting article at the Mozilla Knowledgebase about Exchange and how to fetch mails using Web Access (as Evolution does IIRC)