Have been using Dropbox quite sometime and it keeps my files in sync on the web and all across my computers. Read more about Dropbox here

Dropbox adds support for mobile devices as well. If you visit the website from any mobile phone, you are redirected to the mobile version of the site, which lets you download your files to your mobile phone. I tried it out on my Nokia N97 and it even lets you update files onto your dropbox online folder (Just choose the normal version over the Mobile version on the page and upload using the basic uploader).  Signup for Dropbox to sync across all your devices.

Dropbox running on my Nokia N97

The Dropbox already supports the Iphone with its native Iphone app which lets you sync files with your iphone as well.