My first favorite is Crayola dry erase crayons. They rub off easy for me, but not for my preschooler, and they are easier for him to not smear than the markers - he's a lefty.
Although, my close new favorite is my wireless printer - HP Officejet Pro 8620. I'm laptop based, and it's so nice to print from the kitchen, the living room, anywhere in the house without having to hook up the printer! Plus, the cartridges are separate so I can replace just the blue and not waste yellow or magenta.