If you're in the UK and want a banking API (including payments and transfers), I'm building http://teller.io/. It's been in private beta about 2.5 months and access will be opening more broadly soon. I couldn't wait for banks to get themselves into gear so I reverse engineered all of their mobile apps, took their private APIs and expose a single unified API through Teller.
So far the RBS banks, e.g. Natwest are in prod. If you bank with them, want super early access, understand it's beta product and will give some feedback: sg <> @ <> teller.io
Here is a video of me cURL-ing my bank account yesterday: https://www.youtube.com/watch?v=wwWccFD0wv8
Here is a CLI one of my beta testers @sebinsua/@nouswaves made for his bank account: https://github.com/sebinsua/teller-cli
So far the RBS banks, e.g. Natwest are in prod. If you bank with them, want super early access, understand it's beta product and will give some feedback: sg <> @ <> teller.io