Off-hand weapon damage bonuses

3.5 Rules
If you wield a weapon in your off-hand, you do not get to apply your full strength bonus to damage from your off-hand attack.  Thus, if you have a strength of 18 and you fight with a long sword and a dagger, you can add +4 for your strength bonus to attacks made with the long sword, but only +2 for your strength bonus to attacks made with the dagger in your off-hand.
You can find this information on page 134 of the Player's Handbook.