The B2 Cloud Storage API is a web API and therefore very similar to Amazon's S3 API.
One of the reasons B2 is considerably cheaper than the rest of the services is it works differently. For example, uploading files doesn't require expensive load balancers in our data center. Instead, we require the developer to make 2 API calls to upload a file, instead of 1 in S3. The downside - our API isn't S3 compatible. However, the upside is pretty compelling - we pass on the savings to you and give you considerably cheaper storage.
We are just getting started. Developers are very excited about B2 and will expect dozens, if not hundreds of tools will support B2 in the coming months. You can find many of these on the B2 integrations page.