We have used RadCaptcha in our site. but it is always wrong the first time, or it doesn't work well.
The code
http://.../Telerik.Web.UI.WebResource.axd?type=rca&guid=9b09e8e7-e7e7-413c-95bb-a384298dd817
The code
<telerik:RadCaptcha ID="captcha" runat="server" ValidationGroup="addcomment" ValidatedTextBoxID="txtCode"> <CaptchaImage RenderImageOnly="true" PersistCodeDuringAjax="true" /></telerik:RadCaptcha><asp:TextBox ID="txtCode" runat="server" />http://.../Telerik.Web.UI.WebResource.axd?type=rca&guid=9b09e8e7-e7e7-413c-95bb-a384298dd817