My Americano recipe came from The New York Bartender's Guide: sweet vermouth, Campari and sparkling water with a lemon peel garnish. It's sort of like a Negroni with bubbles, though sweeter without the gin. The bitter/sweet balance is quite pleasant, in fact.
The drink was created by Gaspare Campari in the 1860s. In Italy, it was originally known as a Milano-Torino as it combined Campari from Milan and vermouth from Turin. The Americano name was an homage to the boxer Primo Carnera, the first Italian heavyweight champion in the United States. The Americano, not the vodka martini, is also the first drink ordered by James Bond in the first of Ian Fleming's original novels, Casino Royale.