Update: As of June 2013, Spanning Sync was overcome by events (changes in OS X and Google made it obsolete) and the developer ceased supporting it.
I use Spanning Sync to synchronize the address book and calendars on my Macs with my Google accounts (yes, that’s plural - I have one Google account, and one account at a Google Apps site). It has been working great, allowing me to use my iMac as the central hub by which everything else just stays in sync.
However, it is not free software, and being a heavy user of GNOME and other open source software projects, I spent some time shopping around for a free solution that performed as well as this, as easily as this, and just failed to find one. It’s the first software purchase that I have made other than Mac OS X in a long time, and I think its been worth it.
Spanning Sync recently (a few months ago actually) began encouraging users to advertise the software by offering a discount to both the referred-to user and the referring user.