When I was in the MWC (a.k.a 3GSM) in Barcelona, I encountered an interesting company by the same of ZYB (and also interesting name…:P )
They introduced a mobile application, that is an addition to your address book/contacts and allows you to have extra features that are related to your contacts and mobile devices do not support. the nice thing about the application is that you can access the features just by a click, and as you where browsing your regular contact list. they had features such as sending an instant message and getting your friend’s location. They also do contact synchronization. out of allot of companies that I saw in the WMC, Zyb really stood out.
When getting back to the office, I checked out Zyb. It seemed that the features they demonstrated in 3GSM where not published, so I used their contact synchronization service, and I was very much satisfied. their technology uses Sync ML and is very simple and user friendly.
You go to the Zyb site, set up your phone, get a SMS with the SyncML settings and start syncing away!
For me, their service is very good. I change mobile devices frequently, in order to test new devices, and also when going abroad. I also have 2 mobile devices, one in the car and one for every day usage. although I have a SIM card and most contacts are stored in the SIM, there are cases when not all contacts are saved on the SIM (if someone sends you a contact by SMS…).Its also very good to backup. You can also edit your contact easily on the web, with a regular keyboard, lazy lazy me 
The nice thing abut ZYB is that you can setup a device once and then sync it, so if you setup several devices, all settings are saved and you can chose what device to sync. - very comfortable
There is also an option to re-set your device settings and resent a SMS with the SyncML settings. This was useful after breaking my SE Z610i. I got a new one and I wanted to sync my contacts, I just chose to sync that device, and set it up. a SMS was sent to my new device and I was ready to go! for some reason all my contacts where duplicated, but they had a solution for it!
Cool feature - Merge duplicated contacts: funny enough, after getting duplicated contacts I found a solution , there was a "merge duplications" option. Using this option you get all the duplicated phone numbers, or any contacts that may be duplicated (also by contact name). You can choose to merge them, on the web site with a comfortable screen and keyboard. after doing that, I synced my device again and every thing was good.
Cool feature - Internet calls: you can also make an Internet/Voip call directly from the web site, to any of your contacts using Skype, by a link. what about calling via JAJAH? well lets wait and see…also for now, the JAJAH Firefox Extension does not mark the contact number, allowing you to make a quick JAJAH Call
Needless to say that they have a privacy policy and your data is safe. you can also create a social network in the site, but I didn’t check that out (yet…). I would like to have an option to sync my Facebook events with my mobile, using SyncML…
To summarize: I recommend this service very much, very simple and user friendly. I have been using if for more than 6 month and I’m very satisfied, and its free