I started using this properly yesterday after having it for years since now they also offer an iPhone app. This appears to be much better value for money than the Google, Apple and Dropbox services.
I just upgraded my account to 25GB Free
Your account has been upgraded
You now have 25 GB of free storage. Thanks for using SkyDrive!
Download for Windows, Mac, iPhone, iPad & Android: https://apps.live.com/skydrive
Upgrade your account as an existing customer: https://skydrive.live.com/#!/ManageStorage?lu=1
You will need to log in to your MSN account, and on that link under storage plans it should say "Free Upgrade" or something similar.
Other plans for new customers:
Free: 7GB (25GB Free if you're an existing user and login and use the service now)
US$10/year: 20GB
US$25/year: 50GB
US$50/year: 100GB