Liguria Bakery (1700 Stockton at Filbert, 421-3786) has thrived for almost 90 years by doing a single thing very well: they make focaccia. Period.
They sell ten different kinds--plain ($4 a sheet), with your choice of eight different toppings ($4 to $4.50), and frozen bake-at-home ($3)--but nothing else (except perhaps the occasional raffle ticket to benefit local schools).
The focaccia is baked fresh every morning. If you get there early, the bread is still warm, and you can join the afficionados devouring it on the streets nearby, or on the benches in nearby Washington Square Park. If you get there late, you're likely to find they've sold out and closed early.