The title says it all.
I'll post some I've found in hopes that some of you have used them and the best will be voted up.
closed as not constructive by Mokubai, 8088, Diogo, Randolph West, Renan Aug 7 '12 at 3:50
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Used this for ages, it even has the Google Calendar Data API for implementation on any site that’s free and has CalDAV support to help you sync events between Google Calendar and Apple iCal or Mozilla Sunbird.
I use it everyday, it's very easy!