I work in medical IT. You'd be surprised how many government sites do similar.

An example would be https://sso.state.mi.us/som/dch/enroll/reg_page1.jsp (You can enter any fake name/email, this is only step one of the registration script. The next page has the captch in question.)

The captcha is plaintext, right on the page. The data from the captcha isn't even sent to the server, it is processed locally via JavaScript.

So, the bots don't even have to do anything, but humans have to input a meaningless number...

    <input type="text" name="inputNumber" class="entry-field" size="5" tabindex="3">

    <!-- ... -->

    document.write('<div id="layerNum" class="verifyNumber" align="center">');
    document.write('<b>'+str+'</b>');
    document.write('<img src="generateGIF.jsp?number='+str+'">');
    document.write('</div>');
    document.write('<input size="5" type="hidden" name="rdNumber"  value="'+str+'">');

    <!-- ... -->

    <input type="submit" value="Continue" name="submit" onclick="return Valid();">

    <!-- ... -->

    function Valid(){
    // ...
            if(chkRandomNumber()){
              return true;
            }else{
              return false;
            }
    // ...
    }

    function chkRandomNumber(){
      str1=document.all.rdNumber.value;
      str2=document.all.inputNumber.value;
      if(str1!=str2){
        alert("Please check and type the number as shown in the box");
        return false;
      }else{
        return true;
      }
    }