Sun is also getting into the cloud computing and storage game with project Caroline.  I have heard Microsoft will also be joining possibly with CloudDB but all that is rumors right now.

The only companies in it now are Amazon and Google is emerging. There are also some competitors like Joyent.  But I imagine there will be many companies propping up that are using Amazon or Google or other clous services as their core product.

Amazon is still far far ahead in offering componentized pieces of architecture such as S3, EC2, SQS etc.