For an open source alternative (and my tool of choice) check out git-annex with s3[1]. joehy has a todo for git-annex and glacier[2] and someone has already submitted the beginings of a patch. When i first heard of Glacier I was excited about the possibilty of using it for a backend to git-annex, but then I read cpercival's discussion of glacier[3,4].

[1] http://git-annex.branchable.com/tips/using_Amazon_S3/

[2] http://git-annex.branchable.com/todo/special_remote_for_amaz...

[3] http://www.daemonology.net/blog/2012-09-04-why-tarsnap-doesn...

[4] http://www.daemonology.net/blog/2012-09-04-thoughts-on-glaci...

EDIT: Removed comment about competition.