How can I synchronise my Outlook Calendar with Google Calendar (preferably using a free/open source tool)?

If you're still looking, I recently found an open source project to do this on CodePlex, OutlookGoogleSync. I haven't tried it yet though.

A small tool to keep the Google calendar in sync with the Outlook calendar (one way: Outlook -> Google). Doesn't need admin rights and works behind a proxy. Works with Outlook 2003 and newer.

For future reference, this seems to a a fork with a lot of recent activity, however it is still Alpha at this time: outlookgooglesyncbidirectional

An even more active project (at this time): Outlook Google Calendar Sync which moved, after the closing of codeplex, to here (github).

The synchronisation is sort of no longer needed. You can simply open your Google calendar in Outlook and overlay it. To do this, go to the calendar settings in Google and click on the ICAL button. Copy the URL.

Then go to Outlook and select the "Open Calendar" button from the Ribbon, then "From Internet". Enter the URL you copied above. It now displays your Google calendar next to your Outlook calendar. Click the little arrow next to the Google Calendar name to overlay it.

Google seems to be deliberately not providing these instructions next to the instructions for other OSes and applications. They must have fallen out with Microsoft, very childish!

I am not sure how to make it work in reverse though (add the Outlet calendar to Google) - I expect this is possible if you know the URL of your calendar on the Exchange server.

I downloaded the free version of gsyncit at: http://www.fieldstonsoftware.com/software/gsyncit3/ If you just need to sync your Outlook calendar to your Google calendar it is FREE*. It took a couple of syncs to get everything up to date.

*Update:

As of November 2014 the unregistered version of GSyncIt has some limitations:

The unregistered version is limited to synchronizing one Google and Outlook calendar, synchronizing 50 contacts/note/tasks entries and does not sync deletes for contacts/notes/tasks entries. Registration will remove these limitations and remove any registration/nag dialogs from the application.