- Download the aforementioned zip file
- Install Spanning Sync, which runs as a System Preference pane
- Enter your Google Account credentials, or go get one (they're free, and if you already have something like a Gmail address, you're good to go)
- Select which calendars from iCal you want to sync with Google Calendar (and if you haven't signed into Google Calendar at least once, do so now)
- Press "Sync Now"
- Sit back and marvel at the wonders of desktop + Google Calendar syncing
As of this writing, a price is yet to be announced, though there will be two plans. The first is an annual subscription rate, while the second is a one-time, lifetime purchase for those who want to get past all the riff-raff. Stay tuned for more details.
[Update: Woops - looks like servers fall down, go boom. Spanning Sync has temporarily closed the public beta to new users due to the veritable river of interest overrunning their servers. If you recall your 'Internets 101' lessons, water and server hardware don't mix well. However, users who got set up before they closed the beta are still in - the doors are simply closed to new users until Spanning Sync can add some equipment to handle the load. Their blog post announcing the bad news also states that they hope to re-open the public beta by Wednesday, so we'll keep our eye on things and let you know when new users can begin signing up again.]