Mithral Body( Monster Manual III, p. 192)
A warforged character's body can be crafted with a layer of mithral that provides some protection without hindering speed or gracefulness.
1st level only, Warforged,
Mithral Fluidity (MM3) ,
The warforged's armor bonus is increased to +5, and the character is considered to be wearing light armor. He now has a +5 maximum Dexterity bonus to AC, a -2 penalty on all skill checks that armor check penalties apply to (Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Sleight of Hand, Swim, and Tumble), and an arcane spell failure chance of 15%.
Unlike most feats, this feat must be taken at 1st level, during character creation. Warforged druids who take this feat cannot cast druid spells or use any of their druid supernatural or spell-like class abilities. Warforged characters with this feat do not gain the benefit of any class feature prohibited to a character wearing light armor.
Without this feat, a warforged character has an armor bonus of +2.