This is a fascinating use of twitter and geo mapping. Snow is coming to the UK from the North tonight and so Ben Marsh decided to ask Twitter fans to tweet the first half of their post code with the tag #uksnow. His app then scans Twitter every minute looking for the tweets.
These then get mapped on to an UK graphic, thus showing in real-time how the snow is progressing across the country.
This kind of social interaction could be used for all kinds of surveys. Such as bird migrations, first signs of spring etc.
It just needs enough people to get involved and clever use of geo mapping.