What clever things have you done with an ASP.NET MVC Action method

The ASP.NET MVC controller action methods are primarily used for handling 'business' operations but it can be used for lots more.

I thought it would be fun to see what creative, useful things people have created actions for that may be practical or useful for others.

Here's my contribution :

Javascript file concatenator - to reduce number of http requests:

    [OutputCache(Duration = 5 * 60, VaryByParam="")]  // DONT USE "None" here *
    public ContentResult RenderJavascript(){

        StringBuilder js = new StringBuilder();
        StringWriter sw = new StringWriter(js);

        // load all my javascript files
        js.AppendLine(File.ReadAllText(Request.MapPath("~/Scripts/jquery.hoverIntent.minified.js")));
        js.AppendLine(File.ReadAllText(Request.MapPath("~/Scripts/jquery.corner.js")));
        js.AppendLine(File.ReadAllText(Request.MapPath("~/Scripts/rollingrazor.js")));

        return new ContentResult()
        {
            Content = js.ToString(),
            ContentType = "application/x-javascript"
        };
    }

Map a route to it :

  // javascript
  routes.MapRoute(
     "js-route",
     "dynamic/js",
     new { controller = "Application", action = "RenderJavascript" }
  );

Refer to it from your master page :

    <script type="text/javascript" src="/dynamic/js"></script>

Be warned I've set a cache for the output, so if you're changing your JS and refreshing the page you might want to disable the cache!

I jsut need to come back and figure out how to gzip it.

* You shouldn't use VaryByParam="None" because that causes the Vary header to be send, which causes the browser to g