One of the pioneers of listing mash-ups on the web, Trulia.com  (disclaimer: I am on their advisory board)  has decided to make their API  available to the public. Its basically a way to tie in their search (ie listing) data for your own use. Here’s the summary from Trulia  and more tech stuff from Tech Crunch .
They had their own interoffice competition  which came up with a bunch of demos, two of which were posted on their web site.
Openness a great idea and I think there will be a treasure trove of uses that will evolve with this API by opening the proverbial can of worms.
My favorite of the demos created in this effort (I repeat: demos) was Plot or Not . In this application, different data sets within the Trulia domain are compared to see if they actually correlate, although it may not be a matter of cause and effect.
The screen shot I showed was (I think) pretty obvious. Average household income does correlate to average listing price.
Trulia assures me these demos were created for entertainment purposes only (they obviously know me too well).