The standard protocols for sharing address books are ACAP, IMSP, and LDAP.
Mulberry supports ACAP and IMSP, but, as of 2005 Sept 30, the company that makes Mulberry is in Chapter 7 bankruptcy. Mulberry is available here:
Infotrope Polymer supports ACAP and is available here:
Lots of IMAP clients support LDAP for reading an address book, but not many (if any) make it easy to write/update your LDAP address book(s).
Another option is to store your address book in an IMAP mailbox. That is what Pine does and is how I share my address book between different machines. I've written about this on my Power Pine page in this section:
Note that Pine runs on just about every operating system, including MS Windows, Mac OS X & DreamHost's Debian Linux shell, and is a surprisingly powerful IMAP client.
Hope this helps,
Infinite Ink ~ http://www.ii.com
Deflexion & Reflexion ~ http://deflexion.com