The Tretyakov is of the most renowned art museums in the world, just like the Hermitage in St. Petersburg. It has a lovely collection of Russian Soviet pieces (on the top floor) as well as contemporary and modern art and many galleries with art from throughout Russia’s history. In addition, the museum exhibits work from well-known artists around the world. Admission is somewhat pricey, so go on the third Sunday of the month if you can when admission to all of Moscow’s museums is free.
Nearby are vendors selling their art and in the underground crosswalk there are also a number of art-stands.