If you work with Microsoft Outlook, Google Calendar or Zimbra but like to use iCal as your organizational tool of choice then YAI (You Are Invited) is an excellent plug-in to simplify your life.
YAI is a simple plug-in for Apple Mail that translates and transfers Outlook, Google Calendar, Zimbra and Exchange style meeting invitations to iCal. This means that without having to open any of the above applications, you can see instantly what you've been invited to in iCal.
Installing YAI is all automated as it runs a script to setup the invite transfer rule. From this moment on, all invites are processed as soon as they arrive and appear in the iCal notification pane. You can bring iCal up to date with invites issued before you installed YAI by simply selecting them in Mail and pressing Cmd-Opt-L. The only downside is that in this unregistered version, you are nagged every time you receive an invite to register YAI.
However, it's a very modest amount to upgrade and for anyone hooked on iCal but that also has to use Outlook, Google Calendar or Zimbra, it's an essential plugin.