config.ru

# Rack file for local Rubygems server, using YARD

require "rubygems"
require "yard"

libraries = {}
Gem.source_index.find_name('').each do |spec|
  libraries[spec.name] ||= []
  libraries[spec.name] << YARD::Server::LibraryVersion.new(spec.name, spec.version.to_s, nil, :gem)
end

options = {:single_library => false}
server_options = {}

run YARD::Server::RackAdapter.new(libraries, options, server_options)

You can use this to make a gems.local site in OS X, using PassengerPane. You'll have to create a public directory, too, because PassengerPane insists on setting DocumentRoot Rails-style.

I adapted this to pull from all gemsets (assuming you use rvm).

https://gist.github.com/1313488