The most commonly used ones are Sonneborn-Berger and Bucholtz.
Bucholtz is the sum of the scores of the opponents you have played against, except the worst if your score is higher than 50%, except the best if your score is higher than 50% and except the worst and the best if your score is exactly 50%.
Sonneborn-Berger is the sum of the products of each of your opponents' score and the score you got against that particular opponent.
Some tournaments nowadays use "synthetic" tiebreaks, such as most blacks, most wins, most wins with black.
does anyone know how these are calculated?