There are a few. I was looking into MBTrading for a friend. I didn't get too far, as my friend lost interest. Seemed relatively straigt forward with a C# and VB.Net SDK. They had some docs and everything. This was ~6 months ago, so it may be better (or worse) by now.
IIRC, you can create a demo account for free. I don't remember all the details, but it let you connect to their test server and pull quotes and make fake trades and such to get your software fine tuned.
Don't know much about cost for an actual account or anything.
