PhpGroupWare is a Web-based groupware application that includes applications like a calendar, e-mail, to-do lists, address books, a trouble-ticket system, a file manager, new headlines and more.
The system provides user accounts, user groups, different levels of user access, events for calendar, and IMAP and POP-3 for e-mail. Plans for more features are in the works.
Offering multiple OSs and database support, this may be what you are looking for. As it's currently under heavy development, you may want to check it out.