fruux - Free service that takes care of your contacts, calendars and more so you don't have to.
Funambol - Funambol is the leading provider of platforms that power many cloud services.
FullContact - Put the power of full contact information into your web or mobile apps.
BirdieSync - Synchronize your Android, Pocket PC, or iPhone with Thunderbird and Sunbird
Google Contacts - Google Contacts is Google's contact management tool that is available in its free email service Gmail, as a standalone service, and as a part of Google's business-oriented suite of web apps Google Apps.
Kontact - Kontact is the integrated Personal Information Manager of KDE.