Well with increase in spamming this facility of captcha has done the magic although people always find some or the other reason to crack a security but for the time being this can be implemented..
Captchator is a very good service which can be combined easily with RoR and implement captcha
Jst few lines of code does the magic for you
in img src call the follwing source http://captchator.com/captcha/image/$yoursessionid
Just like the one below
Write the above lines in views which will display a random captcha on the form or the page
Then http://captchator.com/captcha/check_answer/$yoursessionid/$answer using this fetch the value which is entered by the user
If the answer is correct, the result is “1”, if not, it is “0”.
You can check the same in ur browser
Try it in your Browser:
1. Go to http://captchator.com/captcha/image/(anything you like) and read the text from the picture.
2. Go to http://captchator.com/captcha/check_answer/(anything you like)/(text from the picture). If you entered the text correctly, you will see “1”.
Thats it..Captcha Ready for ur site..Aviod spamming in 10 mins no more security and coding required.