I love darning socks. Mostly because I love my wool socks and refuse to give them up. Darning is like the duct tape of sewing fixes. It's inexpensive, durable, and stays put.
Darning wool is easy to find. JoAnn's carries it in the US and both Spotlight and Lincraft carry it in Australia. You'll need tapestry needles, darning needles, or those kid plastic needles, anything with a large eye. You can use a darning mushroom inside your sock, but an orange or lightbulb will do.
It's a lovely process of creating a fabric by weaving back and forth across the hole and into the fabric to recreate the missing area.
Here are some lovely sites to show you how to darn your socks. You can also use this technique with sheets and cottons by sewing back and forth on your sewing machine and then again up and down.
How to Darn Socks
Make Do and Darn