Mac OS X 10.6 Server
Apple has made much of how Mac OS X 10.6 ( Macworld rated 4.5 out of 5 mice ) is a tuning of the operating system, removing older cruft, but not really adding new features to the OS other than some plumbing upgrades and Exchange 2007 support. The same cannot be said about Mac OS X 10.6 Server, however. The latest version of Apple's server OS boasts rather a lot of changes, designed to help boost Server's attractiveness in the small-to-medium business, (SMB) market. Oh, and like Mac OS X 10.6, Mac OS X 10.6 Server is Intel-only.
While not perhaps as dirt cheap as Mac OS X 10.6's $29 upgrade pricing, Apple has both simplified your options for Mac OS X 10.6 Server, and cut the price. The ten-client version is gone, and now there is one option: Unlimited. So no more odd simultaneous access limitations on some file sharing and other services. You want Mac OS X 10.6 Server, you get Unlimited. Along with simplifying the options for Server, Apple cut the price in half, from $999 for Unlimited to $499. Get more, pay less. Wait, is this Apple?
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If Apple had only cut the price in half, and kept the core OS improvements for Mac OS X 10.6 Server, it would be an OK upgrade. But that's not the case by a long shot. Apple has added quite a few brand new features that move this from an OK upgrade to an excellent one. There are some minor issues that keep it from being outstanding, but those issues won't affect everyone.
Address Book Server is a new way of dealing with the problem that networked contacts can create. Mac OS X Server has had a global contact list for some time now, thanks to the LDAP back end for its Open Directory service. But LDAP is really not suited for the kinds of things a lot of companies want to do nowadays. While LDAP is great for a "master" contact list (a global address list [GAL] in Microsoft-ese), if you want to allow people to make custom networked address books, or shared networked address books, it gets squishy. First, LDAP is read-optimized. It's really good at letting you look up and get information from even a huge number of records in a hurry. But it was not designed to have hundreds or thousands of users modifying it throughout the day. It's tedious to properly secure LDAP implementations so that only those who should be modifying it and it's terribly easy to make a mistake that affects the entire directory.
To deal with this, Apple has Address Book Server, which is based on the CardDAV IETF Draft. Just like CalDAV is a set of iCal-specific extensions to the WebDAV standard to make group calendaring easier without being tied to a specific vendor (a la Exchange or Notes), CardDAV hopes to do the same thing for Contacts. Address Book Server implements CardDAV in front of Open Directory, so that users can create their own server-side contacts that exist outside of their local Mac, and can be used by Mail, iCal, etc. Since CardDAV is based on WebDAV, providing secure access outside of a company firewall is no harder than securing any HTTP-based service, and by acting as a limited-access front end to your LDAP store, Address Book Server helps you keep your LDAP data safer from prying eyes.