This is a 3x2 Gridfinity Mouse Organizer. Is made of two pieces, the main piece, and a second one to improve the finish of the slope, you can glue it or just let it fall in place (I haven't glued mine).

There is an small box to drop the USB wireless receiver if you need to keep it organised alongside the mouse. 

Originally designed for Logitech G703, but can fit most if not all mice out there.
 

Is not stackable.