For calendars, you can safely use iCal, Mozilla Calendar and Ximian Evolution (all supporting the .ics calendar format). All you need is a WebDAV server to host them to allow publishing and sharing. To view calendars on a web page, you can use phpicalendar on the server. Easy setup and works very well.
There are various open-source webmail options available. I have played a bit with PHP-Nuke (wich is a complete web portal system that includes some sort of webmail).
For address book, I have never researched that but I imagine googleing for LDAP will provide some insight.
On the palm side, that may be a little tricky on the PC/Windows side. Those using Apple can use iSync to sync their calendars and address book. On linux/unix, Ximian Evolution will do the trick. I don't know if there is anything on the PC/Windows side to send .ics events to a palm.
If you don't have any specific requirements on the PC side, you might as well dump Windows altogether and work with Linux, OpenOffice and Ximian Evolution. These will interact perfectly with your Macs (palm, ical, etc.).
Good luck!
Colddiver