Warforged's Shield (3.5e Equipment)
From D&D Wiki
This masterwork heavy steel shield is specifically crafted to clip onto a Warforged's arm (or a Greater Prosthetic Limb) while leaving the hand free for other tasks. Any attempt to use a weapon in your shield hand, while possible with this shield, is at a -2 penalty to attack rolls that stacks with all other penalties (this penalty drops to -1 if the shield is made instead from mithral). You also take a -2 penalty of skill checks that specifically require the use of that arm (Climb, Swim, etc.) in addition to the shield's normal armor check penalty due to the shield getting in the way (this likewise drops to -1 if the shield is made from mithral). The price is the same as that of a masterwork heavy steel shield make the regular way (170 gp). A Mithral version costs 1,020 gp.
If fighting with two weapons with this shield, you loose your shield bonus to AC unless you have the Improved Shield Bash feat. Fighting with a two-handed weapon or ranged weapon causes you to loose the shield bonus regardless. Spellcasting, while subject to spell failure, may be done with your shield hand, allowing the odd Warforged cleric or spellsword to cast spells while armed and armored.
Note for DMs: if you wish to allow it, you could rule that due to the off-hand being left free to move, the shield incurs 5% less arcane spell failure (down to 10% (or none if it's mithral)).