bits.py

def each_bits(size):
    if size == 0:
        yield ()
    else:
        for bit in xrange(2):
            for bits in each_bits(size-1):
                yield (bit,) + bits


if __name__ == '__main__':
    import sys
    size = int( sys.argv[1] )

    for bits in each_bits(size):
        print ''.join( str(b) for b in bits )

bits.rb

def each_bits(size, &block)
  if size == 0
    yield []
  else
    (0..1).each do |bit|
      each_bits(size-1) do |bits|
        yield [bit] + bits
      end
    end
  end
end

if $0 == __FILE__
  each_bits(ARGV[0].to_i) { |bits| puts bits.join '' }
end

FIGHT!.txt

$ time ruby1.8 bits.rb 16 > /dev/null

real    0m9.861s
user    0m8.977s
sys     0m0.872s

$ time ruby1.9 bits.rb 16 > /dev/null

real    0m1.742s
user    0m1.740s
sys     0m0.004s

$ time python bits.py 16 > /dev/null

real    0m1.227s
user    0m1.204s
sys     0m0.016s