Well, ok, I think this mostly has its own section because I happen to have a Russian Doll sitting on my actual bookcase. It's one my mum brought back for me from Russia, and it's part of a small collection I realised I've built up since I was small ;P
Russian Dolls are otherwise known as Matryoshka Dolls or, perhaps more accurately, Nesting Dolls seeing as a lot of them aren't actually Russian at all. A 'traditional' Russian Doll will consist of 5 individual dolls, although many dolls are available in different numbers (like the one on my bookcase which actually has 10), and no doll should be the same as another.
Here are some links if you want to find out some more information about the dolls, or if you want to buy one (I haven't bought one online from anyone of these people, & I'd recommend doing your own research before you decide to buy. I just chose these ones because they have good pictures!)
The History of Russian Dolls
Online Shop in the UK - pritty pictures :D
Online Shop in the USA - a lot of background info here too