I developed capture plugin to substitute for reCAPTCHA. I used idea of "Very Simple Anti Bot Registration (VSABR)" mod of FluxBB in Q2A. However, I made program in scratch. This plugin works like reCAPTCHA. Question patterns exist in the language file. You can add patterns without limit. It will be useful on none English / multilingual site. In addition, because I added SHA1 certification hash in form, I think security level and bot block performance is high relatively.
1.6.x, 1.7 (2014/12/20 Updated)
I translate Japanese of FluxBB. There are various kinds of bulletin board. But, FluxBB is very fast. People who are also interested in bulletin board should try it. Surely you will be surprised at god's speed.
Have fun !!