How do I remove all HTML tags from a string without knowing which tags are in it?

You can use a simple regex like this:

public static string StripHTML(string input)
{
   return Regex.Replace(input, "<.*?>", String.Empty);
}

Be aware that this solution has its own flaw. See Remove HTML tags in String for more information (especially the comments of 'Mark E. Haase'/@mehaase)

Another solution would be to use the HTML Agility Pack.
You can find an example using the library here: HTML agility pack - removing unwanted tags without removing content?

You can parse the string using Html Agility pack and get the InnerText.

    HtmlDocument htmlDoc = new HtmlDocument();
    htmlDoc.LoadHtml(@"<b> Hulk Hogan's Celebrity Championship Wrestling &nbsp;&nbsp;&nbsp;<font color=\"#228b22\">[Proj # 206010]</font></b>&nbsp;&nbsp;&nbsp; (Reality Series, &nbsp;)");
    string result = htmlDoc.DocumentNode.InnerText;