"Willow water" is a term for water that has had willow soaked in it to extract compounds that improve the chances of a cutting growing roots. Here's a fairly descriptive article on it.
I tried it once when I was given some cuttings from a ficus tree, and one cutting grew roots (it's now about a metre tall, and makes a nice indoor plant).
What I don't know is how much of a difference it really makes to use willow water rather than just plain water (or commercial rooting hormone). Does anyone have a good feel as to how much it improves the strike rate of cuttings?