You should consider Ultimus...
" After a long search for the right business process management platform that would allow us to integrate and extend with .NET, my team selected the Ultimus BPM Suite. The primary factors in the decision were the comprehensive nature of the solution which would allow us to deploy process management without requiring the use of some other tool such as InfoPath or SharePoint.
Having initially found and recommended the product, I was even more impressed with the product as we went through detailed training that brought out a number of features and illustrated an architecture that gave me even greater confidence in the product. This was particularly true in the area of integration points with existing systems and the ability to extend the process using custom developed controls or even process context aware ASP.NET pages hosted outside of the process server.
If you are looking for a better way to deliver business process management and enterprise human-centric workflow solutions, you should consider Ultimus. There were other systems that were much more expensive that may have fit our requirements, but this was the only one we found that allowed us the freedom to extend and customize using our .NET dev skills without requiring coding skills to design and modify and manage processes."