Knockback is a feat, you must be large, and if you hit hard enough they get bull rushed automatically when you hit them. Awesome with the Dungeoncrasher ACF for Fighters. For a good example of a build specialized around this concept, consider Lion Totem Wolf Totem Whirling Frenzy Barbarian 2/Dungeoncrasher Fighter 6/War Hulk 10/X2, where X is pretty much anything you want (I recommend Martial Rogue for the extra feats). Wield a +1 Valorous Skillfull Spiked Shield, buy a permanent Enlarge Person, and go nuts. On a human your feats would be Power Attack, Cleave, Improved Trip (free from Wolf Totem), Improved Bull Rush, Knockback, Improved Shield Bash, Shield Charge, Shock Trooper, Shield Slam, and whatever else floats your boat.
Now whenever you charge, you hit everyone within reach with a full attack (so they all get hit by all the attacks). Anyone hit by this is tripped (vs your incredible strength score) and dazed (save based on strength) and knocked back, and if they hit anyone else that person is tripped too. Also, anyone who is knocked back and hits a wall takes a ton more damage, and you already do insane amounts of damage, and anyone you trip also gets hit a second time... and that extra hit also hits everyone else in range against. So it looks like the following:
Imagine there are three medium enemies next to each other with a fourth behind them (a classic meat shield wall in front of a caster), and you charge them, like so:
Tank1 Tank2 Tank3
You're big, hence so many yous. Anyway, because of Lion Totem you have pounce, and with Skilfull you have a BAB of +14 at, say, level 19. So you have three attacks. The first attack hits all of them due to War Hulk. First, let's calculate Tank 1. You hit him, which trips (Shield Charge) and dazes (Shield Slam) him and knocks him back (Knockback). Before he can move, you get a second attack against him (due to Improved Trip) but this attack hits everyone else again. Now he goes flying back right through the Wizard (Shocktrooper's directed bull rush) which gives you an attack against the Wizard too... unless the tank dies first (which he well might) giving you a cleave attack. You can see how ugly this gets. Enemies just go flying. If you wield a +1 Skillfull Spinning Sword as well, you can make attacks against people who get tripped by Directed Bull Rush pushing people into them, even if they're out of range of your shield. Whee!