Array#rand
これは便利。
require "activesupport" puts ["hoge", "fuga"].rand #=> hogeとfugaのどちらかがランダムに表示される
追記
Array#rand is deprecated and will be removed in Rails 3. Use "random_element" instead
さらに追記
Array#random_element is deprecated and will be removed in Rails 3. Use Array#sample instead.