This is only the result of a quick search but it surfaced mightily interesting projects and interesting initiatives. I am glad to see some open source picking up in the financial applications area. It will be initially hard to compete with some of the software companies in the arena but it may help revitalise internal development efforts in major institutions and enhance the experience here.

Also there is certainly an opportunity there for smaller institutions to leverage open source and participate in it rather than buy very expensive, over the top commercial solutions.

QuickFix is a Fix engine that works with C, Java and even Ruby,

Marketcetera is an order management system that uses mostly java but also has a rails application,

JFin is a java project that offers date manipulation (fixed income developers know how precious this is) and is trying to build a derivatives model,

And a few quant libraries QuantLib, KQuant and JQuantLib.

I also need to mention OpenAdaptor, which was actually developed and contributed by Dresdner Kleinwort.