InvisibleCaptcha
http://haacked.com/archive/2006/09/26/Lightweight_Invisible_CAPTCHA_Validator_Control.aspx
Sample Usage (DEMO)
<script runat="server" language="C#">
protected void OnButtonClick(object sender, EventArgs e)
{
if(Page.IsValid)
{
successLiteral.Visible = true;
}
}
</script>
<p class="success">
<asp:Literal ID="successLiteral"
runat="server"
Text="Valid!"
Visible="false" />
</p>
<div>
<sbk:InvisibleCaptcha id="commentValidator"
runat="server"
ErrorMessage="Oops! You must be bad at math."
Display="dynamic"
ValidationGroup="Invisible" />
<asp:Button ID="invisibleDemoButton" runat="server"
Text="Submit"
OnClick="OnButtonClick"
ValidationGroup="Invisible" />
</div>