One of Rome's most refined examples of Renaissance architecture, the Palazzo della Cancelleria was built, possibly by Bramante, between 1483 and 1513 for Raffaele Riario. Although his great-uncle, Pope Sixtus IV, made him a cardinal at the age of 17, Raffaele didn't allow his ecclesiastic duties to cramp his style. He is said to have raised a third of the cost of this palace with his winnings from a single night's gambling. He was involved in plotting against the powerful Florentine Medici family; in retaliation, the palace was confiscated for the church when Giovanni de' Medici became Pope Leo X in 1513. It later became the papal Chancellery and is still Vatican property (one of the offices housed here is the Rota romana, which grants or denies marriage annulments).
There is a lovely courtyard. The palazzo incorporates the fourth-century church of San Lorenzo in Damaso and, as archaeologists discovered in the 1940s, sits atop a mithraeum and part of a canal connecting the Baths of Agrippa - located just north of largo Argentina - to the Tiber. These ruins are usually visible with permission (06 6988 5318, fax 06 6988 5518), but at the time of writing were closed for further digs.
From time to time chances arise to enter the palazzo. You might be lucky enough to find tickets for the occasional chamber-music concerts held here; alternatively look out for exhibitions on religious themes mounted in the magnificently frescoed rooms.