$ gem install rubygems_plugin_generator

$ gem plugin foo

#       create  foo/foo.gemspec
#       create  foo/Gemfile
#       create  foo/lib/foo/version.rb
#       create  foo/lib/rubygems_plugin.rb
#       create  foo/lib/rubygems/commands/foo_command.rb
#       create  foo/test/foo_command_test.rb
#       create  foo/Rakefile
#       create  foo/.gitignore
# 
# *****************************************************************************************
# Please edit foo.gemspec with your plugin information.
# The file lib/rubygems/commands/foo_command.rb was created for you, there you will
# find a method called 'execute'. That's the method that will be called when someone runs
# 'gem foo'
# *****************************************************************************************

$ rake test
Run options: --seed 52142

# Running:

.

Finished in 0.011959s, 83.6190 runs/s, 83.6190 assertions/s.

1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

$ gem build foo.gemspec

#   Successfully built RubyGem
#   Name: foo
#   Version: 0.1.0
#   File: foo-0.1.0.gem

$ gem install foo-0.1.0.gem

# Successfully installed foo-0.1.0
# Parsing documentation for foo-0.1.0
# Installing ri documentation for foo-0.1.0
# Done installing documentation for foo after 0 seconds
# 1 gem installed

$ gem foo

# It works!