WideNoise is an iPhone app made by WideTag for measuring noise levels wherever you go. You can use it to take a noise sample of your neighborhood and upload the data to the WideNoise website to show it on a map.
I just tried it and apparently I live in a place with a 'sleeping cat noise level' at the moment. Probably because it's quite late in the evening, and the only thing I clearly hear outside is an airco installation on the building next to mine, and some cars passing in the distance.
Once I uploaded my noise sample data within a second the map on the WideNoise website showed a notification about my measurement. But now I wonder what happens next. It would be nice to be able to see a more accurate heat map showing the average noise levels at places all over the world, but that's not available. Maybe more people need to use the app to accomplish this.
I'd say it's an interesting idea but the processing and representation of the captured data needs some work. It could be surprising to see how noisy most of our cities are. Noise pollution is a serious issue.