If you've got a Windows Live ID, then you get 500Mb of storage for free. You can then create personal, shared, and public folders and you decide who has access to each folder.
Private folders are for yourself. Shared folders are for you and your selected Windows Live contacts and you can allow any of those contacts to add or delete files as well. Public Folders are accessible by all, no Windows Live ID required, but are read-only.
Windows users can use Internet Explorer 6 or above and Macintosh users can get access via Firefox 1.5 or later. I've had a play around with it and it is pretty easy to use; pity there's only 500Mb on offer. Maybe once it's out of beta, we'll see an increase to more realistic capacities if they want to compete with other providers. For example: MediaMax give you 25Gb for free, the BT Digital Vault comes with 2Gb free, Google's Picasa Web Albums give 1Gb and divShare is unlimited.
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