Aside from providers that have developed their own solution (Amazon, Nirvanix, Rackspace, etc.), many of the "cloud storage" offerings from hosting companies are based on one of the following commercial platforms:
Certainly EMC is the most stable company here; it appears Parascale may be on the ropes:
Since you're cost sensitive and coming in with a very low amount of storage, I'm not sure if any of these will fit your needs with regards to pricing (though Parascale appears to have a free version on their site for up to 4TB of storage). There are a few open source alternatives, though you probably won't find much production usage of them yet.
Cleversafe Community Edition (*)
I have read something about a company called TwistedStorage having an open source cloud storage offering, but I don't know much about them.
Another thing to keep in mind is that you're really talking about two different types of storage here. "Cloud storage solutions similar to S3" is tier 2 storage, not tier 1. You wouldn't want to run VMs off tier 2 storage.
(*) Disclaimer: I know the founder/CEO of Cleversafe and had some discussions regarding the product very early in it's development, but I have no vested interest in it.