Can't believe I forgot to review Krypton. I have yet to find a captcha plugin that comes anywhere near as great as Krypton. Free and open source, zero bloat, has exactly what a captcha plugin needs and no more.

I wish good future for this plugin! Bots can't bypass it like usual captcha, because it doesn't support authme and it's good ( only for me ). Messages say to players first you need to complete captcha, then register. And bots, they do not understand it, first of all they register but they can't do it because krypton doesn't allow it, they need to do captcha first, but really they do not need to do it because bots are awful. Anyway, thanks to plugin's author. Now my server doesn't have bots.

Simple and easy. I hope its been updated to lasted versions
And got new features :)