Microsoft SkyDrive is a powerful tool which makes it easy to sync and store your files online.
The new system offers 7GB of online storage space, a big drop from the previous 25GB. However, you can now synchronise the full 7GB; the desktop app allows you to work with files up to 2GB in size (the web version and other apps restrict file sizes to 300MB); and the whole syncing system is now much easier to use.
Essentially the program takes a Dropbox-type approach, adding a custom SkyDrive folder to your PC or device. If you copy files to the folder, they'll be synchronised. Rename a file on your phone and the change will appear on your PC, too. Delete a file on the SkyDrive.com web interface and it'll similarly disappear from the folder, immediately.
There's even a new Fetch feature, which means that if you've gone somewhere and realised you've forgotten an important file on your home PC, then you can access it through SkyDrive.com.
And if you need more than the standard 7GB storage, then don't worry. If you were using more than 4GB as of April 1st 2012 then Microsoft have already opted you in to 25GB of storage. If you had uploaded any files to the service as of April 22nd then you can manually opt in to keep 25GB yourself. And if all else fails then you can purchase one of their paid plans, which seem fairly reasonably priced (+20GB for $10 a year, +50GB for $25, +100GB for $50).
Note that the latest SkyDrive includes a basic build for Mac users, enabling you to access SkyDrive from the Finder.
See a post on Microsoft's "Building Windows 8" blog if you need more details.