I have a fairly clean ASP.NET MVC project structure. However, I'm struggling on how to organize the mass of classes which are DTOs (data transfer objects), e.g. just to encapsulate post data from forms (viewmodels) but don't represent full domain objects or anything near that yet; and then the many "result" objects I have which communicate complex result information from my service layer back to the controller. Where do you stuff these/how do you organize them? I have one folder with well over 60 classes now and it's getting cluttered. Appreciate suggestions!
Domain objects should live in a separate Domain Model library. Anything that supports the Domain Model in an framework-neutral way (e.g. no references to ASP.NET MVC, WCF, WPF etc.) belongs in the Domain Model.
Classes that perform translation between the Domain Model and the specific interface framework (ASP.NET MVC in your case) belongs in that particular project (your ASP.NET MVC project).
You can have your mappers etc. in a separate Mappers folder, but personally, I think it is much more valuable to structure code along features instead of infrastructure.