I use Dropbox and would love to have it for Windows Mobile. I did a Google search to see if it supported W.M. and came across this
https://www.dropbox.com/votebox/Mobile%20Dropbox
Please vote for it.
If you don't know what Dropbox is it's a cross-platform cloud-based storage application and service operated by Dropbox, Inc. The service enables users to store and sync files online and between computers and share files and folders with others.There are both a free and paid service, each with varying options.(via Wikipedia)