I've been using Question2Answer for a number of years now to power Ask Planet Kubb. I like Question2Answer because it is simple to run, uses a standard PHP + MySQL setup and can even be used in a multi-site farm configuration easy enough. However, I've been frustrated by it’s ability to manage spam registrations. Question2Answer has built in support for ReCaptcha and it has a plugin method to use other anti-spam measures. I've used ReCaptcha for a while, but it results in 4 to 10 spam registrations a day. I tried switching to the Q2A Logical Captcha by amiyasahu which uses Text Captcha. I even submitted a pull request to properly salt the captchas. But this resulted in 3-4 times the spam registrations as ReCaptcha! Ouch!
I decided to apply the same type of spam defense I've successfully used for MediaWiki and early results are great. It uses the Numbers_Words PHP package to spell out a number of nine digits and then ask the user to identify the number in a specific position. The question looks like this:
This works really well. You can grab q2a-number-captcha and
git clone it for your own use.