Like a darning mushroom but specifically for socks!

Works best for socks in size EU 38-42 (~ US 8-11 and UK 5-8). You can scale it up/down for larger/smaller socks.

You can use the groove on the bottom to hold the sock in place with an elastic band.

I love using it for repairing socks because you can also get to the sides where it's typically difficult to pin a normal darning egg in place. Another advantage is that you don't have to move the darning tool around until the entire sock is fixed.