Do you ever have a blog / forum / personal site where there are instances where your comment forms are filled with unnecesary advertisements, not mentioning horrible languages. For your information, this isn't done by a person, in most cases it will be spam bots automatically generated by a program.

Captcha is well known to prevent or reduce the amount of spams being posted on your blog / forum / sites, most open source PHP CMS such as Drupal / Joomla / Wordpress obviously have a captcha module or even integrated captcha by their own, so don't worry about them. How about free forum such as PHPBB and Simple Machines, they do have it as well, but there are certain tweaks and modifications needed to ensure your captcha is not being bypassed.

From my own experience, the phpbb captcha by default isn't very stricted and easily bypassed, you have to visit their official website to gain a better and stricter captha system. For those of you developing your own site from a scratch, i would strongly recommend using 3rd party captcha system from:

http://www.phpcaptcha.org/try-securimage 

http://www.recaptcha.net         

Commercially, i have been using phpcaptcha in WWWShop websites (the company where i work for at the moment),  it  does seem to filter well and very strict in a sense, and most importantly, a descriptive documentation in regard to installation and how-to-use manual is provided in the official website.