Macs can easily coexist with Windows PCs in most Windows networks. Apple provides built-in support for accessing shared folders and printers on Windows servers as well as for using Active Directory accounts for logging into Mac OS X workstations. One area where Macs don't integrate quite so easily is with Microsoft Exchange Server.
Exchange and Outlook are mainstays in many Windows networks. Exchange provides not only an email server but company-wide contact, calendar, and task management solutions. Both Apple's Mail application and Microsoft's Entourage can access email account on an Exchange Server, but neither product provides access to Exchange's other features. Most Mac users rely on Exchange's Outlook Web Access feature to view shared calendars and tasks and typically rely on their own address book instead of Exchange's Global Address List.
For those Mac users who want better access to Exchange, Snerdware produces AddressX and GroupCal. AddressX syncs the contents of an Exchange server's Global Address List to Mac OS X's Address Book application. GroupCal syncs data in iCal with Exchange calendars and public folders. Like Mail and Entourage, Snerdware's tools rely on the WebDAV functionality of Exchange's Outlook Web Access to function.
An interesting point of trivia is that Mac users weren't always kept on the outside of Exchange services. Back in the days before Mac OS X, Microsoft actually produced a full featured version of Outlook for Mac that offered access to the same features available from Outlook for Windows. Unfortunately, the product was never migrated to Mac OS X.