If you want to use Captcha gem and got this error :

MissingSourceFile

no such file to load -- openssl

/usr/lib/ruby/gems/1.8/gems/activesupport-1.3.1/lib/active_support/dependencies.rb:147:in `require'
/usr/lib/ruby/gems/1.8/gems/captcha-0.1.2/lib/captcha.rb:21

Soluce : sudo apt-get install libopenssl-ruby

In a general way, you need to install these packages to get captcah fully functionnal :
libopenssl-ruby libgd-ruby1.8