Open APIs are one of the hallmark features of Web 2.0 services. They allow developers to tap into external data and repurpose it, creating a new tool "mashed-up" from two previously separate ones.

The easiest to understand examples illustrating the potential of mash-ups are map mash-ups.  Much noise has been made of GoogleMaps Open APIs, because this has led to a huge stampede of map mash-ups that harness the massive amounts of data available from Google.  A great example is chicagocrime.org, a nonprofit that has used crime data together with Google maps to make Chicago crime information freely available in a wide variety of views and layers.  Another example is "Following the Dollars," a mash-up that maps the campaign contributions of individuals within a zip code.

The potential for open APIs is huge.  They offer both application functionality and access to valuable databases, typically at no cost to other developers (consider Flickr, providing access to a huge quantity of tagged photosets).  They allow researchers to cheaply and easily manipulate and analyse previously separate datasets (consider the geographic mapping of apartment rental data provided through the Craigslist Open API). They allow advocates to present the result of an analysis in an intuitive presentation that is far more compelling than a presentation of statistics. The map mash-up examples are perfect examples.What are the best examples of mash-ups that support advocacy and social change? If we could encourage developers to provide open APIs to their tools, which ones would be the most valuable - given the needs of social change organizations?  And what is the optimal balance between free access to data, and market or financial incentives that serves the needs of nonprofits, as well as companies and tool providers?