fruux - Free service that takes care of your contacts, calendars and more so you don't have to.
GoodSync - GoodSync provides highly reliable file backup and synchronization for both individuals and businesses.
FullContact - Put the power of full contact information into your web or mobile apps.
Syncthing - Syncthing replaces proprietary sync and cloud services with something open, trustworthy and...
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.
rsync - rsync is a file transfer program for Unix systems. rsync uses the "rsync algorithm" which provides a very fast method for bringing remote files into sync.