git-annex can already be used to store data in several cloud services: Amazon S3, rsync.net, Tahoe-LAFS, The Internet Archive.
I would like to support as many other cloud services as possible/reasonable.
- Dropbox (I had been reluctant to go there due to it using a non-free client, which I have no interest in installing, but there is actually an API, and already a haskell module to use it. Would need to register for an API key. http://www.dropbox.com/developers/start/core. Annoyingly, Dropbox reviews each app before granting it production status. Whoops my interest level dropped by 99%.)
Post others in the comments. --Joey