Online Banking is implemented - in Germany (HBCI) (Score 1)

The German banks use an online banking protocol (yes, a *protocol* -- not only a file format) that is publicly available, called HBCI (HomeBanking Computer Interface). Guess what? It's already implemented in Gnucash through the OpenHBCI library. I can download transaction statements *and* make online money transfers right from inside Gnucash now.

Point is, online banking (just as the bank systems in general) is totally country-dependent, which limits both your developer and your user base. For countries which fortunately have some openly available standard it is possible to implement this in an OpenSource project.