Climatevo.com - weather statistics and history data
I was always wondering, what was the weather exactly one year ago, with climatevo.com it is (or soon be) possible.
Climatevo.com provides day by day weather information, and average climate data for almost all cities over 50k habitants.
First quest was to get the database of all countries and cities, it was not that hard, but other data (like GPS, translates, and population) was just tedious.
Second was to collect the weather data - and still doing this, every day we get more.
Anyway, try it out.