WebDAV
WebDAV allows people to share files over a http server. This can be used for subversion repositories but also for ical standard calendars in Outlook with the publish to internet / webdav server option.
In basic form it's an insert of the module in apache which you then activate:
<Directory /export/home/wip/websites/Admin/Calendars>
DAV On
</Directory>
This page shows some of the basics and security is discussed in the comments
This discusses how to set up webdav with LDAP support for authentication
NB Make sure the lockfile and directories as well as the directories and files which are web-dav-ed are writeable to the webserver user.
To export a calendar in Outlook to the Webdav server, first create a new calendar, then rightclick on the calendar name in the navigation pane (to the left). There you can choose 'publish to internet' and 'publish to webdav server' under Share.... In the above example the Location would be http://my.internal.ext/Admin/Calendars
Outlook automatically updates the calendars every time you send/recieve. In advanced publishing settings, make sure the update frequency checkbox is off though. When you recieve an invitation for a calendar, also make sure the checkbox for frequency is off and the attachments are on. From Microsoft:
By default, this calendar will be periodically updated. Changes to the calendar are published to the WebDAV server during the next manual or automatic send/receive. By default, there is a 30 minute interval between each send/receive for each Send/Receive group. If you change the automatic send/receive interval to less than 20 minutes, calendar updates to the WebDAV server are sent at a minimum 20 minute interval.
NOTE: Webdav calendars can be shared, but the other users looking at the calendars can't write to them!
some more MS stuff on webdav publishing here
This pages has more info about the free / busy options
If you just want everyone to see them for scheduling purposes, then click Tools->Options->Calendar Options->Free/Busy Options and fill in the amount of information to publish and the update frequency
2013
To create a new calendar, rightclick on an existing calendar and choose new calendar, don't rightclick on a calendar group!
To add someone elses calendar, rightclick on Other Calendars (the calendar group) and select Add Calendar -> From Internet.
Input http://my.internal.ext/Admin/Calendars/filename.ics
Select 'advanced' and uncheck the update limit and check the attachments box.
2011
Publish your outlook 2011 calendar on a webdav server