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.