The Horde Project is an open source project that I've been involved with over the last few years - after adapting my php IMSP classes for use within the Horde Framework. Since then, I've contributed to a number of the Horde projects and Framwork packages. Horde is a PHP based solution that provides, among many other things, Mail, Conatacts, Calendars, Virtual File Storage and Task Lists. All are built apon the sturdy Horde Framework and can connect to many existing backends such as IMAP/SQL/IMSP/LDAP/Kolab. Leverage your existing backends with a great collaboration tool! Check out the website, download the packages and try it out. There is a very active, friendly developer community, reachable through the mailing lists, that is available to help answer questions. There is also paid consultancy available for those with urgent, specific needs.