What are some good sources of data that most people don't know about?

I remember my first job in a real estate agency in 2000 - if you'd told me that one day we could hover over a map and see house sale prices and an automated valuation, I wouldn't have believed you.

Plenty of organisations, associations, and government departments publish great data in secret - sometimes just without the fanfare it warrants, and sometimes surreptitiously because they are required to even if they don't want to publish.

What are some of the meaningful or cool data sources you've used or uncovered in the past?