Data store for testing data products?
You might try Azure Table Storage.  Since you can't lock yourself down to a specific schema (since one data product might be aggregates whereas another might be time series or something else), Azure Table storage would give you the flexibility of storing data from multiple sources, each having their own format.

This would also lend itself to making a system highly scalable, as you could use Azure Service Bus in conjunction with Azure Table Storage.

You might check out this tutorial at Pluralsight, Applied Windows Azure, as it shows a number of examples, one using Table Storage and Service Bus, another using Hadoop, and I suspect that some of these might match the extensibility you are looking for.