GooSync is a free option that works without installing any application (over-the-air). You just set up an account, give GooSync access to your Google Calendar and configure your phone to work with GooSync. GooSync works with mobile devices that support SyncML, and these include Blackberries, most Nokia phones, many Sony Ericsson phones and others. For Palm or Windows Mobile phones you'll have to install a Sync ML client, which is not free.
Besides synchronizing the events, GooSync tries to keep the reminders in sync. Because Google Calendar has some predefined intervals for reminders, this will not work if you define custom reminders for your phone's events. The service's main limitations are that "you are only able to synchronize your primary Google Calendar and you are limited to a sync window of 7 days past and 30 days future of the current date," but if you pay a subscription these limitations are removed.
If you don't mind installing a Java app and synchronizing your calendar manually, then GCalSync is for you. The software is open-source, doesn't have GooSync's limitations regarding the sync intervals and it works pretty well in a wider array of phones. Unfortunately, it doesn't seem to be developed anymore.
For Windows Mobile phones, you could try OggSync, which is compatible with Outlook, it lets you define an interval for synchronization and supports over-the-air and/or cradle sync. Like in GCalSync or the free version of GooSync, you can only synchronize the main calendar, unless you pay for a yearly subscription.
Another Windows Mobile solution is SyncMyCal, which lets you synchronize all your calendar, but the date range selection is limited to 3 days. The paid version of SyncMyCal is much more affordable than OggSync as there's no yearly subscription.
| || GooSync || GCalSync || OggSync ||SyncMyCal|
| Price || Free / $39.79 per year || Free ||Free / $29.95 per year|| Free / $25 |
| Supported phones || Phones that support Sync ML || Phones that run Java apps || Windows Mobile ||Windows Mobile|
| Software || No || Yes || Yes || Yes |
|Configurable sync window|| Paid version || Yes || Yes ||Paid version|
|Scheduled synchronization|| Yes || No || Paid version ||Paid version|
|Multi calendar|| Paid version || No || Paid version || Yes |
|Reminders|| Yes || No || Yes || Yes |
|Recurring events|| Yes || Yes || Yes || Yes |
|Google Hosted Domains|| Yes || ? || Yes || Yes |
Here's also a comparison of the number of blog posts that mention the sync tools, according to BlogPulse (April-June 2007):