Experimental Sitecore ChildActionRenderer - like ControllerRenderer, but using the HtmlHelper Action extension method for rendering. Standard MVC child action behaviour wrt TempData. Has dependencies on the Sitecore.MVC.Contrib project.

ChildActionRenderer.cs

using Sitecore.Mvc.Common;
using Sitecore.Mvc.Presentation;
using Sitecore.Mvc.Extensions;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web.Mvc;
using System.Web.Mvc.Html;
using System.Web.Mvc.ExpressionUtil;

namespace Sitecore.Mvc.Contrib.Presentation.Renderer
{
    public class ChildActionRenderer : Mvc.Presentation.ControllerRenderer
    {
        private readonly IPageContext _pageContext;

        public ChildActionRenderer() : this(new PageContextWrapper(PageContext.Current))
        {
        }

        public ChildActionRenderer(IPageContext pageContext)
        {
            _pageContext = pageContext;
        }

        public override void Render(System.IO.TextWriter writer)
        {
            string controllerName = this.ControllerName;
            string actionName = this.ActionName;
            if (!controllerName.IsWhiteSpaceOrNull() && !actionName.IsWhiteSpaceOrNull())
            {
                var value = RunChildAction(actionName, controllerName);

                if (value == null)
                {
                    return;
                }
                writer.Write(value);
            }
        }

        private MvcHtmlString RunChildAction(string actionName, string controllerName)
        {
            var helper = GetHtmlHelper();
            
            // Important note:
            // The Action extension method requires that a route exists in the route table
            // that maps the controller and action. This is standard child action behaviour.
            return helper.Action(actionName, controllerName, _pageContext.RequestContext.RouteData.Values);
        }

        protected virtual HtmlHelper GetHtmlHelper()
        {
            ViewContext current = ContextService.Get().GetCurrent<ViewContext>();
            return new HtmlHelper(current, new ViewDataContainer(current.ViewData));
        }
    }
}

GetControllerRenderer.cs

using Sitecore.Mvc.Contrib.Presentation.Renderer;
using Sitecore.Mvc.Pipelines.Response.GetRenderer;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Sitecore.Mvc.Contrib.Pipelines.Response.GetRenderer
{
    public class ChildAction : GetControllerRenderer
    {
        protected override Mvc.Presentation.Renderer GetRenderer(Mvc.Presentation.Rendering rendering, GetRendererArgs args)
        {
            Tuple<string, string> controllerAndAction = this.GetControllerAndAction(rendering, args);
            if (controllerAndAction == null)
            {
                return null;
            }
            string str = controllerAndAction.Item1;
            string str2 = controllerAndAction.Item2;
            return new ChildActionRenderer { ControllerName = str, ActionName = str2 };
        }
    }
}

Sitecore.Contrib.ChildActionRenderer.config

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>   
    <pipelines>
      <mvc.getRenderer>
        <processor patch:before="processor[@type='Sitecore.Mvc.Pipelines.Response.GetRenderer.GetControllerRenderer, Sitecore.Mvc']" type="Sitecore.Mvc.Contrib.Pipelines.Response.GetRenderer.ChildAction, SitecoreMvcContrib" />
      </mvc.getRenderer>
    </pipelines>
  </sitecore>
</configuration>