To use this code, require 'sudoku/solver', then call the solve function with a String like

53  7    
6  195   
 98    6 
8   6   3
4  8 3  1
7   2   6
 6    28 
   419  5
    8  79

and the solve function will return a String like

534678912
672195348
198342567
859761423
426853791
713924856
961537284
287419635
345286179

You can run the tests with: ruby test.rb

The original evolution of this solution can be found at http://gist.github.com/41670