Ways to wield a two-handed weapon in one hand:
1) Phalanx Fighter (limited to polearm/spear and shield, must be appropriately sized)
2) Dorn Dergar (with Dorn Dergar Master feat)
3) Lance (while mounted)
4) Titan Mauler (arguably, still counts as two weapons worth of "effort" so attacking with it still eats your off-hand attack; see below)
5) Redcap (Small Fey with Heavy Weapons ability can wield Medium weapons without penalty; wield a medium one-handed weapon still as a one-handed weapon instead of two-handed; see below)
6) Tiefling (Oversized Limbs variant, same as Redcap, but for a Medium creature with Large weapons)
7) Ogrekin Template (Oversized Limb deformity)
8) Quarterstaff Master (Quarterstaff only)
9) Size Step-down (Wield a 2-h weapon one size smaller than you and you wield it as a 1-h weapon with -2 to attack; see below)
note 1: Regarding Titan Mauler, its wording states you wield a 2-h weapon "in one hand" which is mechanically different from "as a one-handed weapon" as denoted in the FAQ. Even though it also carries an explicit exception to default rules by stating you treat it as a one-handed weapon for strength bonus to damage, power attack, etc, making an attack with it is still making an attack with a two-handed weapon so it may subsume your potential off-hand attack as well. May be debatable. Be aware.
note 2: Regarding Redcap, Tiefling, and Ogrekin abilities, normally, when you wield a "wrong-sized" weapon, you step the handiness up or down. A medium creature wielding a large longsword (1-h weapon) wields it as a 2-h weapon with an attack penalty. Now Titan Mauler's Oversized Weapons ability singles out just the attack penalty being countered, but does nothing for the size step-up and, by RAW, if the step-up takes it higher than 2-h or a step-down takes it lower than light, it becomes unwieldable. However, these three abilities for Redcap, Tiefling, and Ogrekin generally call out "penalties" rather than singling out "attack penalty". Moreover, in the stat block for the Redcap, it is given a Medium Scythe, which is a base 2-h weapon and, given step-up, would normally be unwieldable for a small creature. But given that it can wield it, the only logical conclusion is that "penalties" in general includes both the penalty of size step-up and the penalty to attack rolls. Hence, the Redcap wields Medium weapons as their original handiness so they can wield a one-handed weapon in one hand where another small creature would have to wield a medium one-handed weapon as a two-handed weapon. Since the other abilities have mechanically the same wording, just swapping out appropriate sizes, mechanically speaking, they should work fundamentally the same.
note 3: Regarding size step-down, this doesn't change the special properties of a weapon. A Medium creature wielding a Small Longspear would still have the benefit of Reach, even though wielding the spear in one hand (albeit, with -2 to attack), though he does suffer from smaller damage dice. You could also have a Medium creature using a Medium Longsword (or other 1-h weapon) as their main-hand weapon and a Small Longsword as their off-hand; it will count for lower TWF penalties and you can use the same WFocus and WSpec for both weapons. Alternatively, use a Large Dagger (or other light weapon) along with a Medium Dagger.
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