limit.rb

#!/usr/bin/env ruby
Thread.abort_on_exception = true
class JobQueue
        def initialize options = {}
                @q = Queue.new
                @per_minute = options[:per_minute]
                Thread.new do
                        loop do
                                before = Time.now
                                @per_minute.times do
                                        Thread.new(&@q.pop)
                                end
                                diff = Time.now-before
                                sleep [60-diff,0].max
                        end
                end

        end

        def add(&pr)
                @q << pr
        end
end

jq = JobQueue.new per_minute:6
loop do
        jq.add { puts "I should be executed max. 6 times per minute" }
        sleep 0.001
end