A 100% server side, web-based replacement for the Exchange/Outlook horror combination. Website at http://phpgroupware.org. Supports LDAP, Cyrus/Courier IMAP mailstore or pop3 for the weak-hearted. Multiple sql backends available, including MSSQL. A vast plethora of plugins have been debianised, including but not limited to: a shared mp3 manager, a shared Napster client (to be dropped), WAP support, homepages, IRC, SOAP, RSS....
debian apt-get install phpgroupware
Warning: these packages are rather broken & out of date - a bad patch on top of a 0.9.13 prerelease (current from phpgroupware.org is 0.9.14.003 (0.9.16RC1 coming soon)). The project recommends and supports the current release files listed on the project download page.
I'll add tricks as I learn them.
LDAP support on Debian