There are numerous playable approaches to achieving many arms.
Savage Species is probably the book best for it, since that has
the already-suggested girallon’s blessing, which will add three pairs of arms but only to a Large-or-larger subject, and then only at caster level 12th, so that takes some finagling. But you can officially permanency it once you’ve handled that.
the insectile creature template, which comes with six arms to begin with (the LA +2 is extremely costly, but still less than most other things that grant more arms permanently and without magic). Buy-off, or a gestalt game where you can put the LA on one side, makes this a vastly better option than it ordinarily would be.
- also note that Savage Species has a ritual for gaining a template you’d normally have to be born with, so it was possible to gain insectile later in your career.
the non-psionic thri-kreen, which only has LA +1 and has four arms.
- again, the ritual above can change your race, so a half-orc could have become a thri-kreen if he really wanted.
an entire section entitled Three or More Hands on page 42, which discusses using more than two hands to wield a weapon. So this half-orc could have considered dual-wielding four-armed axes instead of quad-wielding two-handed axes. Each extra hand adds another +½Str to damage.
Multiweapon Fighting, Improved Multiweapon Fighting, and Greater Multiweapon Fighting feats. Though note these feats are printed in numerous places, and always requiring the 3.0e feat Multidexterity, so some adaptation for 3.5e is necessary. Really, there is no particular advantage to separating Two-Weapon Fighting et al. from Multiweapon Fighting et al. in my opinion. Particularly awkward for people graduating from dual-wielding to quad-or-more-wielding.
Other places to consider include the totemist from Magic of Incarnum and the psychic warrior from Expanded Psionics Handbook, both of which can achieve many, many limbs—but for natural attacks rather than wielding weapons. Eberron also had a spare limb magic item somewhere, but it doesn’t allow weapon-wielding either.
Finally, polymorph magic could take on one of these forms, or even more powerful forms, without having to pay LA for it. Polymorph effects are probably some of the most broken effects in the game, and getting a whole bunch of arms is far from the most powerful thing they can do, but it is a powerful thing they can do.
Anyway, more arms are powerful, but they’re also very costly unless you’re abusing polymorph effects. LA hurts far, far more than it was given credit for, so both the non-psionic thri-kreen and the insectile template hurt a lot for what they do. Permanency leaves you painfully vulnerable to dispel magic, but ultimately that’s a far lesser cost. Once you’ve done it, though, more attacks, or larger damage multiplier on your Strength, are quite valuable and can deal a ton of damage. Definitely a workable character concept.