What you're referring to here is done with hardwood floors that are NOT tongue-and-groove. If you have this type of floor it's not advisable to stuff ANYTHING in the gaps. The correct way to fix gaps in this type of floor is to take up the floor in winter and re-lay it.
On the other hand, if the flooring is NOT T&G, then go ahead and stuff the neighbor's cat in there. I would stick with a material that's going to 'move' with the floor. This includes expansion as well as contraction.