1. Tonnarelli is a square noodle often made with eggs; many Roman cooks prefer tonnarelli for the peppery pasta dish called cacio e pepe (see Cheese and Pepper Pasta) and for partnering with certain tomato-based sauces.
2. The hollow noodle known as bucatini, a dried pasta made from durum wheat and water, is among the most classically Roman shapes; in the dish known as bucatini all'amatriciana (see Bucatini all'Amatriciana), it is paired with a tangy tomato sauce.
3. Gnocchi, whose name literally means "little lumps," are dumpling-like pastas made, variously, from potatoes, semolina flour, or even spinach and ricotta; in Rome you'll find plump gnocchi like the ones above, as well as gnocchi alla romana, which are usually flat disks or squares (see Gnocchi Alla Romana).
4. Spaghetti can hardly be called a regional food anymore, but in Rome it is quasi-sacred, especially in that luscious cheese-and-egg-sauce pasta dish, spaghetti alla carbonara (see Spaghetti alla Carbonara).