Today I visited the Basilica of Santa Maria Maggiore, it was built in 432 by Pope Sisto III, and it's the oldest and largest Basilica dedicated to the Virgin Mary. There's a museum and archeological sites that can be visited. The Basilica holds the remains of Gian Lorenzo Bernini, and as part of the Loggia tour you can admire the Bernini staircase, a helical staircase that will make your head spin! Also you can closely admire the mosaics in the loggia, which are the work of Filippo Rusuti and the center of the mosaics display an image of the Cristo Pantocrator. The museum holds relics from the Holy Crib, the Holy Cross, as well as from Christ Crown of Thorns.
Entrance to the Basilica is free, but if you want to tour the Loggia, the Archeological site and the Museum it will cost you 11 euros. There are separate tickets for each area in case you don't want to do all three.