The Vasari Corridor, which is essentially a series of hallways built atop the Ponte Vecchio bridge in Florence, is not open to the general public. You've got to book it through private tour companies. But that said, it's the coolest side-trip I've ever taken. If you're an art history fan DO THIS... you book it through any of several tour companies which operate in partnership with the Uffizi. The Vasari Corridor belongs to the Uffizi Museum.
This is a corridor designed by the Renaissance and Mannerist painter, architect, and biographer Giorgio Vasari at the behest of his patron Giorgio Medici. The Medici had purchased a fortified castle on the other side of the Arno river (today it is the Pitti Palace) and wanted to join it to their homes atop the civic heart of florence (which, as today, were in the Palazzo Vecchio). Vasari's solution was to build this long tunnel over the existing structures which lined the Ponte Vecchio, which is the most famous bridge in Florence. If anything else was there he bought it, tore it down, and continued his corridor.
This private walkway spanning half the length of the city was necessary because over the years quite a few attempts had been made to kill the Medici men whenever any of them were out in public. This particular guy, Giorgio, was especially vulnerable b/c he represented a return to Medici rule after a generation of Florence being a Republic. Some of these attacks over the years were fairly successful; the Medici suffered casualties... but ultimately it made them a shrewd and careful family. Thick as thieves.
So this corridor, which begins inside the Palazzo Vecchio, passes through the Uffizi galleries, and then spans not only the 2 or 3 blocks from the museum to the river but also the entire length of the Ponte Vecchio. It's massive. And although it is no longer in use by the Medici it is definitely still in use by the Uffizi. It houses the largest collection of self portraits by great artists in the world. As if you need me to tell you: this is very, very cool! Along the walls hang the likenesses of nearly every great artist, from every great epoch between Mannerism and Post-Modernism. It's a treat to see self portraits by such a plethora of Masters working in so many styles and centuries.
It begins with Vasari and spans centuries. There are autograph self likenesses by Bellini, Allori, Carracci, Dolci, Cranach, Holbein, Pontormo, Titian, Andrea del Sarto, Rubens, Van Dyck, Velazquez, Bernini, Reni, Ricci, Gainsborough, Reynolds, David, Ingres, Corot, Monet, Sargent, Picasso, Hunt, Zorn, Rauschenberg... the self portraiture on display is exhaustive and amazing. Conspicuously missing is a Durer (they have a copy of one of his self portraits) or any of the masters of the high renaissance. But from Mannerism forward, they've got you (and the walls) covered.