Implement the divisibility-by-7 rule
Golfscript, 27 22 bytes

{.9&gt;{.10/\10%2*-f}*}:f


You can use it this way:

1000f


Explanation

{.9&gt;{.10/\10%2*-f}*}:f
{                  }:f    # Define block 'f' (similar to a function)
 .                        # Duplicate the first value of the stack
  9&gt;{            }*       # If the value on top of the stack is greater than 9 then the block is executed
     .10/\10%2*-          # Same as nb/10 - (nb%10 * 2) with some stack manipulations '.' to duplicate the top of the stack and '\' to swap the the first and second element of the stack
                f         # Execute block 'f'


5 bytes saved thanks to Dennis !