I swapped my Captcha over to Recaptcha and had a 99.9% reduction in spammers getting through
A few sites use simple questions from a simple list.. (perhaps even relevant to the site)
When I was looking at recaptcha I though about possibly using a click on picture where you selected two or three items in a row on the same picture but soon realised that would get complicated as you'd need to map out various pictures..
Bots though use OCR to recognise the letters, perhaps something as simple as a recognition picture, for example show them a boat and expect 'boat' as the passphrase, or show them a ball and expect 'ball' as the passphrase..