The Dragon and the Bear, for Ars Magica, already mentioned.
GURPS Russia for Gurps 3e, also already mentioned, is a very good general product, covering all genres of RPGs (as is the way with Gurps) and has suggestions for superhero gaming, fantasy, historical, etc. It has value as a reference even if you don't play Gurps. I'm not sure how it works with the current Gurps edition.
For 3rd edition D&D, there's a lot of material.
Check out, if you can find them, products by Monkey God Enterprises and in particular by Michael Trescia (sp?) He's sort of a slavic RPG specialist. He wrote a couple of modules: Tsar Rising and the Dancing Hut, and a sourcebook called Frost and Fur, a decent 3.5 sourcebook for 'colder' realms and has material for Slavic, Nordic, and Indigenous cultures. It has several classes such as the Vedma, Koldun, Cossack. The Monkeygod sourcebooks were set in a Russia-inspired setting called Torassia, but there was never a sourcebook for that setting. I think the books may all be on RPGnow on PDF.
The same author wrote a Russia-inspired country for the Iron Kingdoms Campaign setting.
Frostburn, from WotC for 3.5 has some select slavic related material in the monsters and spells sections as I remember but is mostly pretty vanilla.
As already mentioned, there's Baba Yaga's Dancing Hut, written for 2e. And of course there's the mention of the Dancing Hut in the 'Artifacts and Relics' section of the 1e DMG, on sale now

.