A simple way to show a popup window

This is what I use:

private static void ShowMessage(Page page, string message)
{
    ScriptManager.RegisterClientScriptBlock(page, page.GetType(),
        "IncorrectDate", @"<script>alert('" + message + "');</script>", false);
}

Here is how to use:

protected void btnSave_Click(object sender, EventArgs e)
{
    //....
    ShowMessage(this, "Success!");
}

PS:Your WebForm must support ASP.Net AJAX (your page must have ScriptManager).