As part of a compliance check for our company, I was required to print the name/version of all FOSS proejcts I'm using.
Most of it was digging around Gemfiles and setup.pys, which isn't that interesting. The neat part was this snippet to extract license information from cookbooks installed on Chef server:

require 'chef'
config_path=File.expand_path("~/.chef/knife.rb")
Chef::Config.from_file(config_path)
rest = Chef::REST.new(Chef::Config[:chef_server_url])
cbs = rest.get_rest("/cookbooks?num_versions=1") # Get all cookbooks, latest version
cbs.reject!{|k,v|k[/^slasher-/]} # Remove my cookbooks
vers = cbs.map{|k,v| # Extract details
  url=v['versions'].first['url']
  deet = rest.get_rest(url)
  lic = deet.metadata.license
  {name: k,license:lic}
}
# Extract to CSV
require 'csv'
csv_string = CSV.generate do |csv|
  keys=[:name,:license]
  csv << keys
  vers.each{|r| csv<<r.values_at(*keys)}
end
puts csv_string

Execute it using chef exec ruby
The result looks something like this:

name,license
rvm,Apache 2.0
yum-mysql-community,Apache 2.0
deploy_wrapper,none
tomcat,Apache 2.0
elasticsearch,Apache 2.0
...
graphite,Apache 2.0
curl,Apache 2.0

The interesting bit is extracting the cookbook list and information using Chef::REST. This is how knife cookbook list does it.