HTML Helpers - ASP.NET MVC's equivalent of JSTL Custom Taglibs

I have found the equivalent of Java's JSTL Custom Taglibs: HTML Helpers.

I started moving logic out of my Razor Views and into HTML Helper classes.

Here's my first, and it's a beauty (uses recursion to build out nested lists):

    public static class CategoryExtensions
    {
        public static IHtmlString CategoryDisplay(this HtmlHelper helper, IEnumerable categories, int indentSize)
        {
            string indentString = "";
            for (int i = 0; i < indentSize; i++)
            {
                indentString += " ";
            }
            StringBuilder stringBuilder = new StringBuilder();
            stringBuilder.Append("
\n");
            stringBuilder.Append(BuildList(categories, indentString));
            stringBuilder.Append(indentString).Append("
");
            return new HtmlString(stringBuilder.ToString());
        }

        private static IHtmlString BuildList(IEnumerable categories, string indentString)
        {
            StringBuilder stringBuilder = new StringBuilder();

            stringBuilder.Append(indentString).Append("  
\n");

            foreach (Category category in categories)
            {
                stringBuilder.Append(indentString).Append("    
\n");
                stringBuilder.Append(indentString).Append("      
\n");
                stringBuilder.Append(indentString).Append("        " + category.CategoryName + "\n");
                stringBuilder.Append(indentString).Append("      
\n");
                if (category.SubCategories.Any())
                {
                    indentString += "  ";
                    stringBuilder.Append(BuildList(category.SubCategories, indentString));
                }
                stringBuilder.Append(indentString).Append("    
\n");
            }
            stringBuilder.Append(indentString).Append("  
\n");

            return new HtmlString(stringBuilder.ToString());
        }
    }

I'm glad to get the logic out of the Razor views (which should not have such complex logic) and into proper C# classes.