Handy Powerattack

[Fax Celestis]

Aside from being a monsterous race, or the Girallon spell I mentioned earlier, what ways are there?

*Thri-Kreen (XPH, UE), Girallon (MM-I), and Domovoi (DMC) have four arms initially. Mariliths (MM-I) have six.
*The Girallon's Blessing spell gives two extra arms and is potionable.
*The Girallon Arms soulmeld gives two extra arms and is obtainable as easily as a two-level Totemist dip.
*The Demonbinder prestige class in Drow of the Underdark allows you to acquire two extra arms via the Marilith bind.
*The Multi-limbed template in the DMG-II gives two extra arms.
*The Insectile template on the WotC website gives two extra arms, though these are contested as usable for regular weaponry.

Further, some graft feats add extra arms, and there are sources I'm forgetting.

To top that off, add a way to get rending damage (such as the Girallon Windmill Flesh Rip maneuver, the Girallon's Blessing spell, or the Two-Weapon Rend feat) and you can multiply your damage exponentially.