I just found this neat CAPTCHA that fits the theme of my forum.
Unfortunately, its written for WordPress, and neither my skill nor free time is sufficient to convert it over for SMF (I'm using 1.1.11 until SMF 2 final comes out).
If anyone else is interested, and converts it, let me know!
http://www.adafruit.com/blog/2010/04/21/resisty-resistor-captcha-solve-the-resistor-values-to-post-a-comment/
Even putting the licence issue aside for a moment, my comments may be of interest.
QuoteTo all the comments about 'bots solving it', it is actually trivial to solve for a bot, don't kid yourself that it isn't.
As for whether it would be worth their time, the question is self fulfilling.
If this is shown to stop spam, more will use it, so it then becomes increasingly attractive to solve automatically, until it's broken.
I'd also point out that it's not possible to use if you're using a text reader and that it cannot be used on US governmental websites because it would violate disability grounds; it's hard to use for someone who doesn't have at least reasonable motor dexterity, and I'd also note that at least 10% of the US populace is considered to have a vision impairment.
It's not a bad idea, but over time it's not entirely workable.
I didn't even beat them over the head with the fact it won't work very well on most mobile platforms which are increasingly becoming common.