ASP.NET MVC3 RESTful Web Service

Just return a JsonResult.
Here is an example I am using in production:

public partial class StudentController : BaseController {
    public StudentController(RESTContext portalContext)
        : base(portalContext) { }

    [HttpGet, Url("organizations/{organizationId?}/students")]
    public virtual JsonResult List(Guid? organizationId) {
        if (organizationId != RESTContext.OrganizationId)
            throw new HttpNotAuthorizedException();

        var query = RESTContext.GetQuery<IQuery<StudentCasesReport>>()
            .Where(x => x.OrganizationId, organizationId)
            .OrderBy(x => x.LastName, SortOrder.Ascending);
        var cases = query.Execute(IsolationLevel.ReadUncommitted);

        return Json(cases, JsonRequestBehavior.AllowGet);
    }

    [HttpGet, Url("organizations/{organizationId?}/students/{studentId?}")]
    public virtual JsonResult Get(Guid? organizationId, Guid? studentId) {
        if (studentId.IsNull())
            throw new HttpNotFoundExecption();

        if (organizationId != RESTContext.OrganizationId)
            throw new HttpNotModifiedException();

        var query = RESTContext.GetQuery<IQuery<StudentCasesReport>>()
            .Where(x => x.OrganizationId, organizationId)
            .Where(x => x.StudentCaseId, studentId)
            .OrderBy(x => x.LastName, SortOrder.Ascending);
        var cases = query.Execute(IsolationLevel.ReadUncommitted).FirstOrDefault();

        if (cases.IsNull())
            throw new HttpNotFoundExecption();

        return Json(cases, JsonRequestBehavior.AllowGet);
    }
}