I was very impressed by this unique CAPTCHA used for registration at www.xero.com.

Basically, you have to play Noughts and Crosses to prove your human!

You have to win the game, which is rigged so that you can do it in 2 clicks.

I liked not having to decipher text - I find CAPTCHAS mentally quite difficult and annoying.

I also like devoting a whole screen to the CAPTCHA. I also liked the fact that I didn't have to face it on the previous page, when submitting the registration form.

The down side is that I had to read the instructions to know how to win, so the whole interaction probably took a about 9 seconds - quite a long time really.

Making a CAPTCHA fun is certainly a good idea IMHO.