Google Maps with heatmap layers of various UK demographics and rates, including crime, population, house prices and religions.
WhoseTurf was an experimental GeoDjango project I made when me and Martin Smith were considering moving to London in 2009. It allowed you to overlay various data sources (population, crime, and other census data) on top of a map, and browse around to examine areas.
It used a custom tile renderer to render heatmap tiles from a PostGIS-backed store of data. The result was... interesting, but ultimately not very useful - overlaying more than one data layer at once wasn't terribly helpful (it tried to make them transparent, but this just resulted in a coloured mess).
In many ways, though, it was the spiritual predecessor to something like http://maps.met.police.uk/, which was released the following year.
The Subversion repository is still available at https://svn.aeracode.org/svn/whoseturf/.