Hey all, I had a little internal project at work, to develop a portal for our customers. Part of it was to accept online payments (echeck and credit cards). I wrote a wrapper around the Authorize.NET AIM integration API and the ECHECK API. The owner of the company gave me the OK to go ahead and open-source it.

http://code.google.com/p/payment-processor/

In the near future I will be adding ARB support as well. The library is fairly simple, but frees web developers from writing all of those tedious POST strings themselves. I wasn't able to find a C# library out there that accomplished this. Anyways, I hope it saves people some time. Cheers!