A few thoughts about brainfuck.

brainfuck.md

A simple C program:

int x = 10;
int y = 10;

if (x == y) {
  x += 3;
else {
  x -= 3;
}

printf("%d\n", x);

A translation of that as a brainfuck program.

++++++++++             |x| store x
>                      |x|0|
++++++++++             |x|y| store y

duplicate x
<[->>+>+<<<]           |_0_|y|x|x| duplicate x
>>>[-<<<+>>>]          |x|y|x|_0_| restore x in the original position

duplicate y
<<[->>+>+<<<]          |x|_0_|x|y|y|
>>>[-<<<+>>>]          |x|y|x|y|_0_|

compare x and y
<<[>>+<<-]             |x|y|_0_|y|x|
>[>-<-]>               |x|y|0|0|_x minus y_|

<+>                    |x|y|0|1|_x minus y_|

[ else
	<-
	<<<---
	>>>>
	[+]
]

<

[ if x == y
	<<<+++
	>>>-
]

<<<.
#

Multiplication is fun!

++++++++++
>+++

<[->>>+>+<<<<]
>>>>[<<<<+>>>>-]    |10|3|0|10|_0_|

<[
  -
  <<[->>>+>+<<<<]
  >>>>[<<<<+>>>>-]  |10|3|0|10|3|_0_|

  <[-<<+>>]         |10|3|3|9|_0_|

  <
]

brainfuck.rb

#!/usr/bin/env ruby

class Brainfuck
  attr_accessor :program

  def initialize(program = "")
    @cells = Array.new(30000) { |x| 0 }
    @instruction_pointer = 0
    @data_pointer = 0

    @program = program
  end

  def step
    print @program[@instruction_pointer] if ENV['VERBOSE']
    case @program[@instruction_pointer]
    when '+'
      @cells[@data_pointer] += 1
    when '-'
      @cells[@data_pointer] -= 1
    when '>'
      @data_pointer += 1
    when '<'
      @data_pointer -= 1
    when '.'
      printf "%c", @cells[@data_pointer]
    when ','
      throw NotImplementedException.new "whoops, not implemented!"
    when '['
      if @cells[@data_pointer] == 0
        while @program[@instruction_pointer] != ']' do @instruction_pointer += 1 end
        return
      end
    when ']'
      if @cells[@data_pointer] != 0
        while @program[@instruction_pointer] != '[' do @instruction_pointer -= 1 end
        return
      end
    when '#'
      printf " |_%d_|%d|%d|%d|%d| ", *@cells[@data_pointer, 5] if ENV['DEBUG']
    end
    raise Exception.new "data pointer out of range: #@data_pointer" if @data_pointer < 0 or @data_pointer >= @cells.length
    @instruction_pointer += 1
  end

  def run
    while @instruction_pointer < @program.length do
      step()
    end
  end
end

if ARGV[0]
  Brainfuck.new(ARGV[0]).run()
else
  program = ""
  STDIN.each do |line|
    program += line
  end
  Brainfuck.new(program).run()
end

Your loops are broken ...

>++++++++[<+++++++++>-]<.>>+>+>++>[-]+<[>[->+<<++++>]<<]>.+++++++..+++.>
->+++++++.<+[>[+>]>]<<<+++++++++++++++.>>.+++.------.--------.>>+.>++++.