As a Mac user at home and a Windows user at work, I had looked for a solution to sync my iCal calendar with my Google Calendar. I did find some ways to do it, such as taking advantage of iCalshare publishing, but never found one that did a two-way sync – that is, changes I make in my iCal or Google Calendar would reflect the same data – until I heard of Spanning Sync.
Unfortunately, it’s not freeware, but for $25/year or $65/lifetime subscription, it isn’t too bad, considering the benefits it gives you.
So check it out! Peace!