Dropbox is a great service for keeping all of your files automatically backed up. If you're looking for a more personalised solution, Instructables user hackitbuildit shows us how to turn a Raspberry Pi into a personal cloud service.
Hackitbuildit uses the open source syncing service Owncloud to control the data, and the Raspberry Pi is set up to with an external hard disk and wireless network card. From there, it's all about setting up the server, and then getting Owncloud installed. The end result is your own server and syncing service that you control so you never have to deal with terms of service or outages.
If you don't have a Raspberry Pi, you can still roll your own syncing service on your own web host. Head over to Instructables for the full breakdown of how to make it yourself.
Raspberry Pi Owncloud (dropbox clone) [Instructables]