A month or two ago, I included a simple but superbly useful device in a round-up of 'ingenious objects'; it was a u-shaped piece of wood designed to slot over your sofa arm and provide a flat surface to rest your drink, book or small plate upon when the table is to far and the floor too low.
It wasn't cheap – over £160. But it was beautiful and despite the simplicity of it, something so useful it almost justified the price tag. In case you didn't agree, I have just found this.
Bankplank, Ep Bordewijk, around £20 (prices are in Euros) plus around £4 delivery within Europe.