Barbarian Might (history)

Note from the Update 7 Release Notes: The Barbarian capstone, "Might", now correctly grants +10% glancing blow weapon damage and 5% chance to produce special weapon effects in addition to its +2 Strength bonus. (A half-orc or warforged barbarian with all three THF feats, Great Weapon Aptitude, and Might will have deal 60% weapon damage with their glancing blows, with a 20% chance to produce special weapon effects).

Here is the breakdown:
 * Two-Handed Fighting feats each add 3% chance to proc. There are 3 feats for a total of 9%
 * Great Weapon Aptitude enhancements each add 2% chance to proc. There are 3 for a total of 6%. (15% total)
 * This capstone adds 5%. (20% total)

Realisticly it can be even higher however, as most Barbarians will take Frenzy Berserker.. Which add another 2% per tier, for a total of 6%. 26% total if you are a Half-Orc or Warforged Barbarian with every related feat and enhancement.