Well, there are other services: Google's Big Table, among others. But the title begs a different, better question: What about a local version of S3?
What would be great is a small engine that would run on my hosted server (and locally) that would imitate both S3 and SimpleDB services.
In your case, you could use it as an additional backup for S3. In my case, S3 would be the backup. Plus the performance (especially of SimpleDB) is killing me. It fails regularly. So this would be much faster and simpler.