I'm the tech lead on the team that runs http://police.uk, the UK Police crime mapping website, and our data is available to download and through an API at http://data.police.uk. The dataset isn't nearly as big as some of the ones in this list (~40MM rows), but it's nice that the Home Office are open with their data.
People have built some cool apps with it which are showcased[1] on the site, there's even a Pebble watch app[2] which is due to be added to that list shortly.
We also have an open-source Python client[3] for the API, which I'm planning to post here once the documentation is finished.