Have you got a hole in your sock/jumper/cardigan/hat/other? Do you need to darn it? Then you need a mushroom! Enokii and shiitake won't help here - these classic wooden darning mushrooms help you to get the tension right on whatever you need to fix, keeping the shape of the fabric and making sure you don't end up making it all wavey. I also find it really helps you to see just where you actually need to put your needle.
Made of wood. Can be taken apart for easy storage or wood glue can hold it together permanently.