If you are using IIS 7, you should look at Dan Atkinson's answer.
I'm using IIS 6, so, in my case, I have the option to install isapi rewrite for IIS 6 or create custom route. I prefer to create my simple custom route class
AndraRoute.cs
// extend Route class,
// so that we can manipulate original RouteData
// by overriding method GetRouteDate 
public class AndraRoute : Route
{
    // constructor
    public AndraRoute(
        string url, 
        RouteValueDictionary defaults, 
        RouteValueDictionary constraints, 
        IRouteHandler routeHandler)
        : base(url, defaults, constraints, routeHandler)
    {
    }
    // get original RouteData
    // check if any route data value has extension '.html' or '.anything'
    // remove the extension
    public override RouteData GetRouteData(HttpContextBase httpContext)
    {
        var data = base.GetRouteData(httpContext);
        if (data == null) return null;
        // from original route data, check 
        foreach (var pair in data.Values)
        {
            if (pair.Value.ToString().Contains('.'))
            {
                var splits = pair.Value.ToString().Split('.');
                if (splits[1] == "html" || splits[1] == "anything")
                {
                    data.Values[pair.Key] = splits[0];
                }
                break;
            }
        }
        return data;
    }
}
RouteCollectionExtensionHelper.cs
public static class RouteCollectionExtensionHelper
{
    public static Route MapAndraRoute(this RouteCollection routes, 
        string name, string url, object defaults, object constraints, 
        string[] namespaces)
    {
        if (routes == null)
        {
            throw new ArgumentNullException("routes");
        }
        if (url == null)
        {
            throw new ArgumentNullException("url");
        }
        var route = new AndraRoute(url, 
                            new RouteValueDictionary(defaults),
                            new RouteValueDictionary(constraints), 
                            new MvcRouteHandler());
        if ((namespaces != null) && (namespaces.Length > 0))
        {
            route.DataTokens = new RouteValueDictionary();
            route.DataTokens["Namespaces"] = namespaces;
        }
        routes.Add(name, route);
        return route;
    }
}
RegisterRoutes method in Global.asax
public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
    routes.IgnoreRoute("Content/{*pathInfo}");
    routes.MapAndraRoute(
        "Product",
        "product/{id}/{slug}",
        new { controller = "product", action = "detail" },
        null, null
    );
    routes.MapAndraRoute(
        "Default",
        "{controller}/{action}/{id}",
        new { controller = "home", action = "index", id = UrlParameter.Optional },
        null, null
    );
}