Located at the bowels of all Ha'tak vessels, Goa'uld shield generators are large glowing devices which provide the energy that the shields are formed from. Despite their importance, they are quite vulnerable, and can be disabled by fragmentation grenades.
When Apophis launched an attack on Earth with two Ha'tak vessels, SG-1 violated orders to stop him. After mining one ship with C-4, Bra'tac moved this ship closer to the other. Jack O'Neill then disabled the shield generators on the other Ha'tak by throwing two grenades into them from a balcony above. (SG1: "The Serpent's Lair")