Ristorante Il Pellicano (near Porto Ercole): This Michelin two-star restaurant at the amazing (but pricey) Hotel Il Pellicano is open for dinner only. The view is breathtaking. We had the tasting menu, which not surprisingly featured mostly fish, with a last savory course of pigeon breast sandwiched around foie gras. Some of the fish courses were great, others were just ok. Service is excellent. Overall, we liked the less fancy restaurant of the hotel (Pelligrill) better.
Pelligrill (near Porto Ercole). Excellent fresh fish (several types) available daily, open lunch and dinner. Also has several appetizer salads and pastas--and chicken and steak mains that we didn't try but that seemed popular. As befits the hotel, however, the prices are still quite high.
La Pinta (Porto Ercole). This small restaurant sits right on the port; unfortunately the night we went they said they had no fish, only shellfish, including one that seemed like a spider crab (the italian word started with an m). We had one order scampi and one order of the crab-looking thing; both reasonably good but the latter somewhat difficult to eat. We had started by sharing an order of linguine with mixed frutti de mari. All accompanied by a locally produced ansonaco white wine. Decent meal but not great.
La Margherita (Isola del Giglio). Excellent all around. Located right on the port, kitchen (located across the street from seating area) smelled deliciously of garlic. We shared a pasta alle vongole (clams) and asked them to throw in some cherry tomatoes as well. We then had a whole fish for two cooked in "acqua pazza" (a spicy broth). A side dish of sauteed green vegetable (sort of like chard or broccoli rabe) and a bottle of ansonaco produced on the island. A great informal meal and not too expensive. And a wonderful island to visit (notwithstanding the surreal sight of the crashed cruise ship Costa Concordia, which they are currently trying to tow to Genoa).