public static MvcHtmlString ShowModalForPage(this HtmlHelper helper, int pageIndex)
{
    WebViewPage page = helper.ViewDataContainer as WebViewPage;
    if (page != null)
    {
        page.DefineSection("Head", () =>
        {
            StringBuilder builder = new StringBuilder();
            builder.Append(@"<script type=""text/javascript"">");
            builder.Append("$(document).ready(function () ");
            builder.Append("{");
            builder.Append("    showModal(" + pageIndex+ " );");
            builder.Append("});");
            builder.Append("</script>");

            page.Write(MvcHtmlString.Create(builder.ToString()));
        });
    }

    return MvcHtmlString.Empty;
}

You still need to return a MvcHtmlString regardless of the fact it's being added to the page via the WebViewPage.Write method.

I've been using StackService recently to create a private REST API for a project at work, and enjoying its simplicity over T-Rex sized WCF alot. I've discovered that it doesn't fit neatly with the pattern we employ at work for API design: certain behaviours are explicity disabled to ensure you don't get the domain into an invalid state (creating a user for example). StackService is designed so that everything is mapped directly to objects or DTOs, so essentially everything you do is based around an object - CRUD operations. If you want a set of custom parameters for a REST method you need to create then a brand new object is required for each method.

That's how it's designed and the author has made it clear why it's like that, which has meant searching for another REST framework that fits our design pattern. I've tried the 2 competing technologies for StackService - WCF and NancyFX. You can expose REST with WCF but it's a lot of farting about for what many people think isn't a pleasant experience, one of the reasons StackService was made. NancyFX was my clear alternative choice, but as I discovered its .NET 4-centric and the projects I'm looking after are stuck with 3.5 for various reasons.

So I did what every software developer does: re-invented the wheel and made my own micro REST framework, called MicroRest. It's sitting on Bitbucket with just 4 classes and not much code, albeit very simplistic reflection. All and any feedback (as long as its glowing and sycophantic) are welcome, as are patches and contributions on Bitbucket.

One thing that isn't mentioned in my answer is how to filter your log messages according to error type: information, error, critical etc. It's fairly easy to do with TraceListeners, but requires quite a few extra lines of XML in your web.config/app.config. The example below shows how to filter information messages, it's setup to use the eventlog and write to the ASP.NET category, which is necessary with web apps if the user you are running the app pool with doesn't have admin access, or the registry key for a new event log source doesn't exist.

<system.diagnostics>
    <!-- Add multiple sources for varying levels of tracing -->
    <sources>
        <source name="debugSource" switchName="defaultSwitch" switchType="System.Diagnostics.SourceSwitch">
            <listeners>
                <add name="eventLogListener" type="System.Diagnostics.EventLogTraceListener" initializeData="ASP.NET 2.0.50727.0" />
            </listeners>
        </source>
    </sources>
    <switches>
        <add name="defaultSwitch" value="Error"/><!-- See System.Diagnostics.SourceLevels enum -->
    </switches>

    <trace autoflush="false" indentsize="4">
        <listeners>
            <add name="eventLogListener" type="System.Diagnostics.EventLogTraceListener" initializeData="ASP.NET 2.0.50727.0" />
        </listeners>
    </trace>
</system.diagnostics>

You should be using a wrapper/Façade class for the logging so you can have a single entry point and class with one responsibility in your app. Checking for the output level is then easy:

private TraceSource _traceSource  = new TraceSource("defaultSource");

public void LogInformation(string message,object params args)
{
    if ((_traceSource.Switch.Level & SourceLevels.Information) == SourceLevels.Information)
    {
    ...
    }
}