Sant'Eustachio Il Caffè (near the Pantheon) is Rome's iconic coffee bar. Tazza d'Oro is the other classic.
Both are stand-at-the-bar experiences. Espresso for €1.10, freshly roasted in-house.
Don't ask for a 'gran caffè' — that's a double espresso, served lukewarm. Just 'un caffè' is what you want.