MobileMe

This service is great.  For $99/year, you have a 20 GB of back up space that is being sychronized with your computer every hour,  You also have access to your email, pictures, calendar, and backed up files on your www.me.com website.  If you have an iPhone, it updates everything from your computer to www.me.com to your iPhone.  From the website, you can find your iPhone and erase the contents remotely if you think it was stolen.