Awe inspriring combination of productivity geekery, apple geekery, and open source geekery.

For Leopard, Apple pulled its iCal development in house, and paired its iCal client with a new calendar server. [...] Apple built a standalone calendar server based on the open CalDAV specification. It also announced plans to release its calendar server as an open source project in the same pattern as the Apache web server.

This strategy allowed Apple to focus specifically on the demands of a calendar server, rather than delivering a single product with a wide scope attempting to do a little bit of everything. It also offers the open source community an alternative to emulating Exchange Server. By offering a standards compliant CalDAV server under the Apache license, Apple can use the best existing email server while also sharing its calendar server to the community and Linux administrators, encouraging the adoption of CalDAV.

-- AppleInsider on iCal 3.0

Just an awe inspriring combination of productivity geekery, apple geekery, and open source geekery.