The recent 10.5.3 update introduced built-in syncing with Gmail Contacts in Address Book.
Despite this, developers of third-party Address Book-Gmail syncing apps are soldiering on. Both SpanningSync 
and Syncman developers point out that 10.5.3 offers this only for Leopard users and, even then, only for Leopard users with an iPhone or iTouch device.
Jeff Nichols, Syncman developer, has just released a new improved version of his sync app, lending credence to his claim that Wateree (his software firm) is a “small and agile company that can adjust quickly to our customers needs and desires”.
Syncman 1.1 can now be configured to run as a menubar utility and to load automatically when you fire up Mac OS X.
Behind the scenes further tweaks have improved the way Google Talk address are mapped to Jabber addresses in Address Book, and improved treatment of how Address Book’s Last Name field is handled.
But the number one request of users was for scheduled syncing, and Syncman delivers on that too.
The Preferences allow you to set the period of the sync and to customise the level of confirmation you want before it makes any changes:
Confirmation is another nice feature of Syncman, that is lacking in Address Book’s default sync option. As Jeff puts it:
Syncman respects the effort you’ve put into maintaining your Address Book, and therefore gets your confirmation before making any changes that could potentially cause you a whole bunch of headache.
So Syncman offers a confirmation dialog displaying potential changes before it makes them:
SpanningSync has also recently launched a 2.0 beta of its software, which is addition to syncing iCal and Google Calendar, will also sync Address Book data, including photos (Syncman is promised to have this feature soon too). The beta is free (but is a beta, so backup!).
SpanningSync costs either USD 25 for a year’s subscription or USD 65 for a once-off, unlimited licence.
Syncman is shareware and costs USD 15 (€9.95). You can get a 30-day free demo from Wateree’s web site.
I use the the Gmail syncing option found in new Address Book and it works without an iPhone, any iPod connected to the machine kicks off the syncronisation of contacts and during that they are synced to Gmail too.
I did notice that the .Mac sync icon in the menu bar (now quite similar to the one used by Time Machine) spins when this task is active, so it likely uses the framework to sync with Gmail. Not sure if you can start it by itself without any iPod attached.
I can confirm that the Gmail syncing is spawned by iSync alone, you don’t really need an iPod/iPhone. It’s true that the tools listed above do much more than the simple syncing.