Module Link: http://drupal.org/project/captcha_after The CAPTCHA After module allows you to only display the CAPTCHA after X number of failed attempts so normal users don't have to deal with CAPTCHAs.
Who: Technical and non-technical users When: Use this module when you want to protect your site from spammers but don't want to trouble your users with CAPTCHAs. Note: This doesn't necessarily protect your site from spam, more from manual brute force attacks. Where: You can modify the settings at admin/user/captcha/captcha/after Why: Asking a user to fill out a CAPTCHA after X failed attempts is not unheard of. It makes sure you are a human and slows you down enough if you are trying to brute force a form.
