free-memory.rb

#!/usr/bin/env ruby
# encoding: utf-8

# By Uğur Özyılmazel, @vigobronx | @ugurozyilmazel
# http://vigodome.com | http://ugur.ozyilmazel.com | http://github.com/vigo

def get_paged_memory_usage(match_string, paging=4096)
  mvar = 3
  if match_string.split(/[^\w]/).length > 1
    mvar = 4
  end
  paged_val = `vm_stat | grep "Pages #{match_string}:" | awk '{ print $#{mvar}}'`.to_i
  gigabyte_val = (paged_val * paging) / 1024 / 1024 / 1000.0
end

installed_memory = `sysctl -n hw.memsize`.to_i / 1024 / 1024 / 1000.0
total_consumed = 0
['wired down', 'active', 'inactive'].each { |key| total_consumed += get_paged_memory_usage(key) }
printf("%.2fG of %.2fG free\n", (installed_memory-total_consumed), installed_memory) # returns in gigabytes

To make the script available as the command free in your Terminal, see the guide at http://tekkie.dev/mac-os/free-m-alternative-for-mac-os-x