Actually the Captcha is custom made and like sgt said all Horizon's server side stuffs are made in PHP.
I actually copied the register form of Horizon when I made my save sharer...
You can find the script here
, it needs to be modified a lot though and you will have to change the mysql query to PDO.
And if you find my save sharer somewhere you can easily decompile it and get the Captcha snippet.