Resolution of Recaptcha Error in Drupal 7 forms
I personally use the Are You a Human PlayThru Module:

PlayThru is designed to be more simple, more intuitive, and more fun than other CAPTCHA options. We use game mechanics to create an experience that’s more enjoyable than figuring out what that awful squiggly text says.
PlayThru is also more secure. Because users interact directly with PlayThru, we can analyze that interaction and track the resulting data to generate a Human Confidence Score: our level of confidence that a user is an actual human.

You can also check out a video about it this module here:
Drupal 7 Are You a Human PlayThru Module - Daily Dose of Drupal episode 78

Update
The Are you a human PlayThru service has shutdown, I recommend you go with ReCaptcha

Uses the Google reCAPTCHA web service to improve the CAPTCHA system
and protect email addresses.
reCAPTCHA helps power massive-scale online collaboration.