Two-Weapon Fighting

Two-weapon fighting reduces the to-hit penalty when using two weapons at the same time. The penalty for your primary hand lessens by 2 and the one for your off-hand lessens by 6, so it becomes -4 (main hand) / -4 (off-hand) (instead of -6/-10 without this feat). If the off-hand weapon is light, the penalties both decrease by another 2 points, down to -2/-2 (instead of -4/-8 without this feat).

This feat also works for unarmed Monks - but has a different effect as monks do not suffer TWF penalties for using their fists anyways. It instead adds additional attack hooks on attacks #2 and #4. Some developer verification of this was posted by Eladrin on the US forums.

This brings the total number of attacks with Two-Weapon Fighting to: as posted by [Eladrin] in the DDO Forums

All Classes Except Monks

Unarmed Monks

Rangers with the Tempest III Prestige Enhancement

Prerequisite: Dex 15.

Special: A ranger automatically gains Two-Weapon Fighting at 2nd level, even if he does not have the prerequisite for it.

A fighter may select Two-Weapon Fighting as one of his fighter bonus feats.

Related Feats:
 * Improved two-weapon fighting
 * Greater two-weapon fighting