Improved Shield Mate (3.5e Feat)
From D&D Wiki
 Improved Shield Mate [General, Fighter]
Prerequisite: Base Attack Bonus +4, Shield Mate(3.5e Feat)
Benefit: Adjacent allies receive a +2 Shield bonus to AC when you are using a Shield with which you are proficient and can take actions. The bonus increases to +3 if you are using a Tower Shield.