Data Cleansing/Enrichment: Which Open Data sets are useful?

Question posted by Phil Caisley:

In the near future, we'll be implementing DQ software to facilitate the cleansing and enhancement of our internal data. Ideally, we'd like to utilise recognised, good quality open source data sets in this work.

I'm interested in the Forum's views about what's available AND useful, e.g. Geonames (http://www.geonames.org/about.html), and views about the quality, completeness and accuracy of any recommended data sets.

Thanks in advance for your guidance/ideas.

Best wishes

Phil