I just discovered that Scheduleworld stopped their free contact syncing service. Although the service worked fine, I didn't think it justified the $30 USD/year subscription service for just my personal addresses.

So it's time to look for alternative solutions. I've looked at several alternatives, that satisfied the following requirements:

• It should sync my addresses from my iphone 'over the air', without the use of iTunes or another PC solution. My opinion still is that the iPhone is a internet device that should not be connected to a computer to make it work.
• It should sync my Thunderbird (addressbook) contacts in some way with my iPhone in a 2-way direction.
• Preferably, the contacts should be stored by a well-known, free or cheap online service, but I'm also willing to set-up a web-application myself if it's not available.

For now I settled for both ZIndus to sync my thunderbird addressbook with Google Contacts, and I use NeuvaSync to sync between the iPhone and Google contacts.

To set it up:

1. Apply for an account with Nuevasync, and set it up to sync with google contacts. You must do this explicitly. I used NeuvaSync before for syncing my google calendar, but I initially forgot to turn it on for contacts as well. Do this by logging on to the NeuvaSync website, and you can see what you enabled.
2. Install the Zindus Thunderbird Add On, which can be found on the  ZIndus Webiste.

One of the drawback I've seen with this approach, is that the address given in Thunderbird cannot be synced in a normal way. As an alternative, Zindus makes some embedded XML out of it. The result is that the address is readable from the iPhone, but it is surrounded by some special XML code. This makes entering addresses on the iPhone difficult to impossible, but you still can sync telephone number and other items. So you should use your Thunderbird addressbook to enter the addresses.

On the possible side, the set-up comes close to 'free' as in beer. Zindus is open source (MPL), and use is made of the free Google Contacts service, which will probably continue to be free for quite a while. So as long as the NuevaSync account will also be free, we have a very nice solution.