Thanks for your interest! Nimbus.io will have public git repositories, "developed in the open" before we ever charge money to use the service. We just haven't posted the links yet. :)
We admire OpenStack and Ceph as great examples of open source S3 alternatives. Also, Riak+Luwak isn't protocol-level compatible with S3 but offers similar capabilities and an truly elegant design.
Nimbus.io takes a different approach than the above options in that it focuses on space efficiency using parity instead of replication, allowing the storage of a little more than twice as much data using the same hardware. It's a tradeoff of cost vs. latency. For long term archival storage, while throughput matters greatly, latency less so. That's why the price is $0.06/GB.