This large brick structure next to the Arch of Septimius Severus, built during Diocletian's reign in the late 3rd century AD, is the Forum's best-preserved building—thanks largely to having been turned into a church in the 7th century. By the time the Curia was built, the Senate, which met here, had lost practically all of the power and prestige that it had possessed during the Republican era. Still, the Curia appears much as the original Senate house would have looked.
Today, the Curia generally is open only if there's an exhibit inside; luckily, that's not infrequent. Definitely peek inside if it's open, and don't miss the original, intricate floor of marble and porphyry, done in opus sectile.
Via Sacra, northwest corner of Foro Romano, Rome, 00186, Italy