Many users find, and I agree, that captcha can be difficult and bothersome to enter. I've seen some great systems that ask simple questions like "What sound does a cat make?" or "What's 1 + 2?" Provided the correct answers they allow you to continue with registration and the process continues. IMO, these are a much more human way of detecting humans, and hopefully one that will cause less irritation.
How difficult would it be for someone to whip up a plugin for this?
I was also going to make a request. I had the perfect one for my phpBB3 board but I just converted to mybb. I still have the forum set up on my server as I'm still fixing little bugs from the conversion. You can see it
here (until I'm finished fixing bugs and remove it). I was getting over 200 spambot registration attempts daily and none ever beat it. It shows images at random so you can't really make a way to beat it especially if you add your own images. Let me know if you need a better explanation of it but I'm sure you'll get it as soon as you see it.
I installed it as instructed, and got this error:
Parse error: parse error, unexpected '(', expecting '}' in /home/content/p/o/e/poetheghost/html/forum/inc/plugins/wnr.php on line 194
You should post help in the appropriate forum.