I have a unhealthy fascination with data graphics. I stumbled across this website a few months ago which features some interesting examples. The above map by Eric Fischer took publicly available data from the Muni (ahem), San Francisco’s Municipal Transportation Agency.
It shows the geographic coordinates of their vehicles to create this map showing average transit speeds over a 24-hour period. While I’m at it, I can also highly recommend this book too, ‘Information Is Beautiful’ (£9.98 from Amazon!).