Good analysis of the existing plugins.
Modern bots are quite sophisticated, and their developers are paid a lot, so they are always trying to get around the latest defense. For that reason I think its good to stick with an option that is actively maintained and worked on, like ReCaptcha. I also think that the users understand the interface and feel safe knowing that you are taking steps to protect their data.
I had to sift through all of the rails captcha options for a project, and wrote up a sample app for my client to test and try out.
simple-captcha-demo.heroku.com
They were all pretty easy to use and set up, and I like using heroku as a test bed to get something set up quickly, and let a client test it out.
I also wrote up some of my experience and gotchas on my blog RailsPerformance.com
There may be new plugins, its always good to see what the trending is on
www.ruby-toolbox.com
