My fighter character has taken the Willing Deformity (p. 125) and Deformity: Teeth (p. 121) feats from the Heroes of Horror handbook. It means that he now has a natural bite attack which deals 1d4 + STR damage.
My Strength is low, so is it possible to improve bite attack damage somehow, ideally staying in the fighter class? Are there any nice feats to take or maybe some kind of "jaw-weapon"?
Without getting esoteric and multiclassing away in search of bite optimisation...
You can use the Improved Natural Attack feat from the Monster Manual to improve the damage of your new natural weapon:
Choose one of the creature’s natural attack forms. The damage for this natural weapon increases by one step, as if the creature’s size had increased by one category...
It probably isn't really worth it in your case, though, since the step up from 1d4 is only 1d6, and you can almost certainly get more value from a different feat choice (compared to an effective +1 damage on a secondary attack).
You can only take Improved Natural Weapon once for any particular attack form (such as "bite") so you can't even stack it up until the bite becomes worthwhile, either. It's mostly useful for larger monsters that already have high base damage, choosing attack forms where they've got multiple attacks, such as claws (most creatures with claws get 2+ claw attacks), or bites for creatures with multiple heads, like a hydra.
Anything which increases your size category - or at least increases your size category for the purposes of determining attack damage - will improve the damage of your bite attack, as per the Increased Damage By Size table - which would again bump you up from 1d4 to 1d6. (It would also have the probably more significant benefit of increasing the damage of any other weapons you are wielding, as per the Larger and Smaller Weapon Damage table.)
The most readily available option for increasing your size is enlarge person, which as a 1st level spell is relatively cheap to acquire in potion or wand format. If you can find further ways to increase or effectively increase your size category which stack together, you'll take your damage even higher, though I'm not sure there are any simple ways to do that without starting to multiclass (or somehow acquiring some templates).
Wearing an amulet of mighty fists will provide a magical enhancement to your bite attacks:
This amulet grants an enhancement bonus of +1 to +5 on attack and damage rolls with unarmed attacks and natural weapons.
Consolation Prize: In lieu of the amulet, a friendly druid or ranger can cast magic fang on you to grant a +1 enhancement bonus for a few minutes.
If your DM allows use of not-updated 3.0 material, Savage Species (page 58) includes the necklace of natural weapons, which allows for effectively enchanting your natural attacks as if they were a manufactured weapon... including that:
For instance, a +1 throwing returning necklace of natural weapons would apply its enhancement bonus and the throwing and returning special abilities to one or more of the wearer's natural weapons.
Throwing, returning magical projectile teeth attacks sound pretty horrifying.
The aberration-focused Lords of Madness (page 46) adds the +1-equivalent magical weapon property Mouthpick Weapon, which enchants a weapon such that a creature with a natural bite attack can wield it effectively by holding it in their mouth (it's designed for creatures that don't have hands, such as beholders):
A mouthpick weapon looks similar to a normal weapon, but when a qualifying creature places the handle of the weapon in its mouth, the handle writhes and transforms into a shape that can be wielded easily in the creature’s jaws. The creature’s natural bite attack is now replaced by the weapon attack...
This replaces the natural bite attack with a normal, iterative weapon attack, and it can be any weapon sized appropriately for the creature. Like a greatsword (or perhaps, for the low-strength/high-dex Mouthpicker, a finesse weapon such as a rapier). Of course, you're now a weirdo running around projecting an entire sword out of your face and headbanging enthusiastically towards your enemies, which might not be the visual you were going for when you decided to have a bite attack. Also, since it replaces the bite with a manufactured iterative rather than natural weapon, you don't get it as a secondary attack any more and have to bodge two-weapon-fighting rules to use it in combination with other wielded weapons.
