Cinnamon is a Gnome Shell fork which uses Muffin as it’s Window manager (a mutter fork). It is the default Shell used in Linux Mint.
Cinnamon 1.6 was released yesterday featuring many new customization options and bug fixes. It now has a smaller code base and a 2D session (suitable only for troubleshooting for now). the new release sees Nemo (a Nautilus 3.4 fork) integration.