One of the coolest mashups that came out of the Music Hackday was definitely Citysounds.fm, built by Henrik Berggren and David Kjelkerud. Citysounds.fm is a really great way of tuning in to the contemporary sound of cities around the world.
The tracks and city information is fetched from the SoundCloud API–in fact the list of cities and tracks is even updated in real time as new tracks roll into SoundCloud. Additionally, pictures for each city are fetched from Flickr.
We should also mention that Citysounds.fm is built with Ruby on Rails using our excellent Ruby Gem. The gem makes it a breeze to integrate SoundCloud into your application. You can read more about it over at Github.