$LOADED_FEATURES

foo.rb

puts '  - required foo.'

class Foo
end

loaded_features.rb

puts "Requiring foo the first time"
require 'foo'

puts "Trying to require foo again"
require 'foo'

puts "Removing foo from $LOADED_FEATURES"
$LOADED_FEATURES.reject! { |feature| feature == '/Users/josh/code/ruby/sandbox/foo.rb' }

puts "Trying to require foo again"
require 'foo'

output.txt

$ ruby loaded_features.rb
Requiring foo the first time
  - required foo.
Trying to require foo again
Removing foo from $LOADED_FEATURES
Trying to require foo again
  - required foo.