First, here is a nifty web tool that generates custom grafitti. Actually it's more of a web application than a mashup. To get the image here I had to do a screen capture and then crop with photoshop.
http://www.graffiticreator.net
And more seriously, here is a great real-time display of the blogsphere as tracked by digg labs. You need to let it run for a while to see it work. The bogsphere often reports on news events sooner than conventional media. This is a new application based on data mining of another web-accessed application. An in-house mashup at digg.
http://labs.digg.com/swarm/