Suppose I have a warrior who wants to wield two long swords, one in each hand (clearly). What are the penalties associated to this combat style, and which abilities can I choose in order to reduce the penalties to a minimum?
In D&D 3.5, if you have the Two-Weapon Fighting feat, your penalties for two longswords are -4/-4. You can reduce this penalty to -2/-2 by wielding a light off-hand weapon.
The feat Oversized Two-Weapon Fighting (Complete Adventurer page 111) allows you to treat a longsword as a light weapon, which would reduce your penalty to -2/-2.
It's possible to reduce the penalties even further by taking the Tempest prestige class (Complete Adventurer p81). At level 2 the penalties reduce from -2/-2 to -1/-1, and at level 4 to -0/-0. You can meet the prerequisites for this prestige class at fighter level 6 and attack with two longswords at -0/-0 by total level 10.
You would take a -6 attack penalty for your "main hand weapon" and a -10 penalty for your "off hand weapon". If instead you weilded a longsword and a shortsword you would take -4, and -8 respectively. The primary way to lessen this is by taking the Two Weapon Fighting feat. It lessens the main hand penalty by 2, and the off hand penalty by 6.
From the D20 SRD (Hypertext SRD)
In 3.0, You could take 2-weapon fighting, ambidexterity, light off-hand weapon. The penalty would be -2/-2. With 2 long swords, it would be -4/-4.
In 3.5 (on page 160) One could take 2 weapon fighting and take a light offhand weapon would have a penalty of -2/-2. If your fighter takes 2 long swords (medium offhand weapon), the penalty would be -4/-4.
Directly from PHB:
main_hand - off_hand (attack bonus penalties).
Normal penalties –6 –10
Off-hand weapon is light –4 –8
Two-Weapon Fighting feat –4 –4
Off-hand weapon is light and –2 –2
Also keep in mind that no matter what, an off-hand weapon's damage rolls receive only half (rounded down) your strength modifier.