From John Maisey:
YAI (You Are Invited) is a plug-in for Apple Mail that translates and transfers Outlook, Google Calendar, Zimbra and Exchange clone meeting invitations to iCal. It corrects many incompatibilities between Microsoft Exchange style meeting invites and what iCal can understand.
With YAI installed Exchange style invites received in Mail's inbox are automatically processed as they arrive and are sent to iCal, ready to be acted on. Most invites will appear in iCal's notification pane, but any invites the user sends themselves will be added as an editable event. If there are invites from before YAI was installed, these are easily transferred to iCal by selecting them in Mail and pressing Cmd-Opt-L. They will be processed in exactly the same way as newly received ones.
The incompatibilities YAI fixes include problems with Exchange style invites from different time zones, invites that generate warnings about the recipient not being on your 'me' card and differences in the way event updates are handled that result in error messages in iCal.