I am excited about the OWIN initiative it really can improve .NET web application development space. Just to test if its possible I've created a sample that demonstrate how to combine Express.js with Asp.Net Web Api. You can find the sample here
When you clone the repo all you need to do is:
and now you're running Express.js and ASP.NET Web API in the same process thanks to excellent edge.js and slightly modified version of owin-connect.
Here are some screens, response from express.js:
response from ASP.NET Web API:
More possible usages here