I use Microsoft's SkyDrive, which is excellent. I also make extensive use of DropBox when working with clients.
To be honest, if all you need is storage so you can have access to files when you move around, any cloud provider will do. If you want to work collaboratively with other people via the Cloud it can get a little more involved, especially if several people are working with the same files.
'The Cloud' is just a fancy name - what you are actually doing is uploading your files to someone else's server, and downloading them again at some point in the future. Don't use The Cloud if all you want to do is archive stuff - it's far better to use removable media for that. If all you need is a few gigabytes of file storage you can use a memory stick and carry it around with you.