What is the best or most interesting use of Extension Methods you’ve seen? [closed]

This is one that’s been getting some play from me lately:

public static IDisposable Tag(this HtmlHelper html, string tagName)
{
    if (html == null)
        throw new ArgumentNullException("html");

    Action<string> a = tag => html.Write(String.Format(tag, tagName));
    a("<{0}>");
    return new Memento(() => a("</{0}>"));
}

Used like:

using (Html.Tag("ul"))
{
    this.Model.ForEach(item => using(Html.Tag("li")) Html.Write(item));
    using(Html.Tag("li")) Html.Write("new");
}

Memento is a handy class:

public sealed class Memento : IDisposable
{
    private bool Disposed { get; set; }
    private Action Action { get; set; }

    public Memento(Action action)
    {
        if (action == null)
            throw new ArgumentNullException("action");

        Action = action;
    }

    void IDisposable.Dispose()
    {
        if (Disposed)
            throw new ObjectDisposedException("Memento");

        Disposed = true;
        Action();
    }
}

And to complete the dependencies:

public static void Write(this HtmlHelper html, string content)
{
    if (html == null)
        throw new ArgumentNullException("html");

    html.ViewContext.HttpContext.Response.Write(content);
}