I know this is ever so slightly anal, but...
I do like a clutter free interior and found the arm rest storage has limited use by virtue of the fact they move up and down which isn't great for loose items. Here's how I've used mine:
It's handy having a road atlas inside the car but they either tend to sit on the floor or flop over the door pockets. I found the Collins fold-out page road atlas (available from amazon etc) to be a perfect fit for the rear arm rest. I can still reach it from the drivers seat too:
For the front I've kept safety in mind and found the small halfords first aid kit (I supplemented it with a resusc guard) fits right in and a life hammer fits perfectly on the lid if you use short fixing screws. I think it's a great place for it because often they are fitted down in the footwell but what if you are the one pinned in by a stuck seatbelt? This way the seatbelt cutter can be reached from where you sit and it has the advantage of not being permenantly on show:
I did tell you it was a bit anal! But thought I would share the idea