Ran into a scenario today where I needed to add a base tag to my page. Couldn't find a clean way to do it with the stock asp.net controls, so created a couple quickies:

public class HtmlEmptyControl : HtmlControl {

	protected override void Render(HtmlTextWriter writer) {
		writer.WriteBeginTag(this.TagName);
		this.RenderAttributes(writer);
		writer.Write(" />");
	}
}

Very simple, just renders an empty element (self closing tag). Code there is borrowed from the HtmlLink control.

public class HtmlBase : HtmlEmptyControl {

	public override string TagName {
		get {
			return "base";
		}
	}

	public string Href { get; set; }

	public string Target { get; set; }

	protected override void OnInit(EventArgs e) {

		Uri requestUri = Page.Request.Url;

		string relativePath = Page.Request.CurrentExecutionFilePath;
		string relativeRoot = VirtualPathUtility.GetDirectory(relativePath);

		UriBuilder baseUri = new UriBuilder() {
			Scheme = requestUri.Scheme,
			Host = requestUri.Host,
			Port = requestUri.Port,
			Path = VirtualPathUtility.AppendTrailingSlash(relativeRoot)
		};

		this.Href = baseUri.ToString();

		base.OnInit(e);
	}

	protected override void RenderAttributes(HtmlTextWriter writer) {

		if (!string.IsNullOrEmpty(this.Href))
			this.Attributes.Add("href", this.Href);

		if (!string.IsNullOrEmpty(this.Target))
			this.Attributes.Add("target", this.Target);

		base.RenderAttributes(writer);
	}
}

A self initializing base tag. Usage like so:

this.Page.Header.Controls.AddAt(0, new HtmlBase());

Which I just threw in my Master Page's OnPreRender. This is useful for cases where the request's PathInfo starts with a / (i.e. page.aspx/foo/bar).