1-on-1 unorganized tournament scoring system
Try this, it's a system called Whole-History Rating. From the abstract:


  Whole-History Rating (WHR) is a new method to estimate the
  time-varying strengths of players involved in paired comparisons. Like
  many variations of the Elo rating system, the whole-history approach
  is based on the dynamic Bradley-Terry model. But, instead of using
  incremental approximations, WHR directly computes the exact maximum a
  posteriori over the whole rating history of all players. This
  additional accuracy comes at a higher computational cost than
  traditional methods, but computation is still fast enough to be easily
  applied in real time to large-scale game servers (a new game is added
  in less than 0.001 second). Experiments demonstrate that, in
  comparison to Elo, Glicko, TrueSkill, and decayed-history algorithms,
  WHR produces better predictions.


It's been used rather successfully by a game I play called Arimaa. For a tournament score, rather than a player skill rating, you will probably want to treat all games as being played simultaneously, as opposed to allowing the ratio to fluctuate over time.

If it's source code you're after, you may find this pure Ruby implementation helpful.  It can support any two player game, as long as the outcome is a simple win/loss.