For a long time, I've been looking for a clean way to keep all my contacts in sync between the multiple platforms I use. Currently, I have a Macbook at home, a Windows PC with Outlook at work, the iPhone and my main email, a Gmail account. 
 
I have tried almost all available solutions to keep them in sync. Plaxo, multiple applications to sync Outlook to Gmail, Mac to Gmail but no success on having a clean way to do it and without hassle.
 
About two months ago, I found a promising one. 
Soocial. Their objective is to have a central platform and from it sync to multiple points via web APIs or local applications.
 
For the last months, I installed it and substituted all sync options I had by Soocial. In the beginning, many bugs were found. Sync between Soocial and Outlook had problems with multiple emails, Soocial-Gmail had problems with work phones (phone did not appear on Gmail) and some minor ones.
 
About 
a month ago, Soocial rewrote their Gmail communication using the newest API and that sync problems went away.
 
This week, Soocial 
released a new Outlook client that finished the problems I found out. Having this, made Soocial the best platform to keep contacts in sync on multiple platforms.
 
Currently the support Mac Address Book, Outlook, Gmail, Windows Live, Yahoo and other phones(Nokia/Sony Ericsson) via SyncML protocol.
 
The topology I choose to have all my devices in sync was:
 
- Soocial - Gmail connection (it's bi-directional)
- Soocial - Outlook connection via it's application
- Soocial - Mac connection via it's application
- iPhone sync directly to Gmail using Exchange Protocol described 
here 
It's been working perfectly and practically online where any update in any point triggers the sync making all devices updated.
Have you experienced any contact sync solution? Leave your feedback on comments.
Disclaimer: This post is not endorsed by Soocial or any other cited companies in any way. All content posted represents my own opinion.