I don't think that you will easily find a real free (by this I mean for any project, any language) hosted CI service because such a service is very CPU, RAM, disk intensive which implies specific rules, hardware, pricing.
For some offers, have a look at Outsourcing Continuous Integration or this question here on SO. I didn't look at all solutions in detail so I don't know if they'll meet your requirements (language, tool and pricing).
Or try to join a forge providing Continuous Integration for open source projects like The Codehaus (EDIT: not an option for .NET projects AFAIK) or CodeBetter. This will certainly require some efforts to get your project accepted (few actually are IMHO) but this might be your best option.