Until now it was almost impossible to get a good overview of your co-worker calendars and appointments integrated with Outlook.
GroupCalendar is an easy to install Outlook add-on, which solves that problem effectively and flexibly. In addition to allowing group viewing of calendars, GroupCalendar also allows you to create and edit appointments on behalf of others. All of these features are available using nothing more then a peer to peer network, LAN or even a ftp server - no server software is required.
Group Calendar is also available as part of OutlookInside. If you need more than calendar sharing... have a look at that.