My team use Rhino Mocks at work, and as a Moq fan, one of my most missed features is the ability to access invocation arguments when returning a value. For example:

mock.Setup(x => x.Execute(It.IsAny<string>()))
    .Returns((string s) => s.ToLower());

Rhino lacks this feature out of the box. It is possible, but pretty ugly:

mock.Stub(x => x.Execute(Arg<string>.Is.Anything))
    .WhenCalled(invocation =>
        invocation.ReturnValue =
            ((string) invocation.Arguments[0]).ToLower());

Today I wrote some quick extensions for Rhino to make it behave a bit more like Moq.

mock.Stub(x => x.Execute(Arg<string>.Is.Anything))
    .Return<string, string>(s => s.ToLower());

Grab them here: RhinoExtensions.cs