Banks and financial institutions may well be the last class of services to offer open APIs. In the meantime developers will have to look elsewhere to get this sort of data and so it's notable that last week the first banking-related API appeared via the innovative personal finance service Wesabe. You can get details on their site and you can view a summary on our new Wesabe API entry on ProgrammableWeb.
As co-founder Marc Hedlund announced on their blog: "We already provide a set of tools for automatically uploading your bank and credit card data to the Wesabe site. With the release of this API, we’re adding tools to help you automatically download and manipulate that data, too. As a result, we’re not just providing an API for our own site, but also for all the bank and credit card sites that Wesabe supports, as well. Since Wesabe supports banks and credit cards in over 30 countries around the world, we’re effectively providing developers everywhere a way to unlock data from their financial institutions and put that data to work."
Since security is a top concern for sensitive financial data like this, it's something the Wesabe team has certainly factored-into the API (just as they have with the service overall). You can check their API security page and the team is actively answering questions from developers such as this blog response from Sam Quigley, "At the moment, there isn’t any special REST security in the API. This version uses HTTP basic authentication and requires everything to be encrypted with SSL (SSLv3 or TLS v1). This is the same (or better) than what online banking websites use, and ensures that all communications are encrypted end-to-end. It’s also a very widely-adopted standard, so it also ensures that your data is easily (and securely) accessible in as many languages as possible."
And folks are already developing on top of this: ace Windows developer Scott Hanselman has already created a C# library for this which you can read about on his blog post Wesabe makes Financial Data available programmatically.