I am interested in Quandl Open Data, from Quandl.com
These data are also denoted as Wiki Data since it relies on users to flag errors. In particular on their website, they say:
This new data source is different because it is “original”; the data is manufactured by us and Quandl users. The definitive version of the data actually lives on Quandl and not elsewhere.
Unfortunately it remains unclear how the term "original" should be intended, that is whether they buy data from exchanges and/or they compare and transform other free data sources.
Since these data has a straightforward API, many quants here might find it a viable testbed, to validate some code or models before going in production and it is likely that the followers of this community are involved with these free historical data and can share their knowledge.