What are these LEGO pieces? Heavy 6x6 disks
They are officially "Turntable 6 x 6 Round Base" AKA NinjaGo Spinners.  They come in sets with Ninja minifigures and weapons that you can then spin up and have them fight one another.

There are a lot of different varieties depending on the specific coloring, they can be found here:

http://www.bricklink.com/catalogList.asp?q=ninjago%20spinner