The choice of beer in Springfield.
Duff is available both on tap (in Moe's Tavern and probably other places), and also in cans as can regularly be seen in the fridge at the Simpsons. We're never really told much about what Duff tastes like, whether it's more of a lager or an ale etc. The Duff brewery also sponsors various things such as the Duff blimp.
Duff has featured as more than just a passing comment in a number of episodes.
Episode 4F15 - Homer vs. The Eighteenth Amendment
An ammendment to the Springfield town charter is found that is basically prohibition. The Duff brewery launches "Duff Zero", thinking everybody will drink it. 30 minutes later, Duff goes out of business.
Episode 9F14 - Duffless
Homer and Barney go on a brewery tour and get drunk. Homer gets arrested on the way home and swears off beer for a month - and more surprisingly, manages to keep to his promise!
Episode 9F17 - So It's Come to This: A Simpsons Clip Show
Bart's april fools joke is shaking up a can so it explodes and puts Homer in hospital.