The Honeypot module is a great captcha alternative, as it keeps spam bots from submitting content while also saving your site visitors from having to type in mundane character combinations.
Configured properly it will prevent the majority of bots from submitting forms on your site including registration forms, contact forms, comment forms, content forms... any drupal forms.
It works differently from Captcha: it lures the bot into filling out a form field invisible to regular users. By doing so, the system recognizes the bot for what it is and denies the submission.
While being very user friendly, this reversed bot detection system comes at the cost of some bot submissions getting through anyway. This is why I would advise against using this module on large sites, where it is difficult to track every piece of submitted content. It should work well for smaller and medium sites however, it has been working well in many of my projects including this very site.
Honeypot configuration
Correct configuration of the module is extremely important, as wrong settings might make the module inefficient or worse, prevent real users from submitting forms. After configuring the module, make sure to double check it works by submitting a protected form as an anonymous user.
Once installed and enabled, go to admin/config/content/honeypot to configure the module.
First of all careful with the "protect all forms" option, as caching will be disabled on every page that includes a form. This can be problematic in cases where e.g. a login block is embedded in the sidebar. In addition to ticking what forms to protect, there are two important settings to keep in mind.