The Kazan Cathedral

Kazansky was built at the beginning of the 19th century during one of the many Russian-Turkish wars. Alexander I decided that building a large duplicate of St. Peter's in Rome would prove that Russia was a serious superpower that Turkey shouldn't mess with. Apparently it worked; the Turks surrendered before the cathedral's completion and it was decided to not build a southern colonnade to match the northern one facing Nevsky.
At the moment the Museum of Religion is housed here.

In socialist times the cathedral housed the ideologically-slanted Museum of Religion and Atheism and had a graphic Spanish Inquisition exhibition in the basement, complete with a pair of legs jutting out of a cauldron. The current exhibition has a small section (in Russian only) on the history of Catholicism and a larger section on Orthodoxy which includes church art, historical paintings, and various religious knick-knacks.

Field Marshal Mikhail Kutuzov, hero of the Napoleonic War, is buried in the cathedral and there are monuments to him and to General Mikhail Barclay de Tolli in Kazan Square, facing Nevsky. Note that from a certain angle, General Barclay de Tolli seems to be doing something that he shouldn't be doing in public; this is revenge on the randy general for sleeping with the sculptor's wife.