My best friend uses iCal, but my sister uses Google Calendar. How do I keep up with their invites?
I recently found the Spanning Sync  application, which synchronizes Google Calendar and iCal and allows you to share calendars between multiple Macs. It also connects Google Calendar to your iPod, iPhone and other devices.
All you have to do is download Spanning Sync , share your Google Calendar, then sync it with iCal on your computer. From then on, any changes you make in Google Calendar show up in iCal and vice versa. When I tried it the sync was quick and effective but the application wasn't 100 percent intuitive. I had to create new "calendars" in my Google calendar in order to transfer over my iCal calendars, a step that isn't clear when you look at the sync page. The application is free for 15 days, after which you can sign up for either a $25 one-year subscription or a $65 one-time purchase.