As recommended by mate who works in tack shop, used to be with the French Olympic team as a groom, I think. It's quite new to the market, contains cedar wood oil for relieving itches, comes in peach, strawberry, original. £14, I just bought some.
I understand neem oil is also fab, made up as a spray and has 'cured' someone's sweet itch horse (in frontline thread in All Aout Dogs section)
Depends what is causing the itching - my Clydie is a real tail itcher and I was hers with Nizoral and it seems to sort it out. Designed for humans with dandruff it seems to clear it really fast.
NAF teatree oil shampoo is the best thing for my horse - he has very sensitive skin and reacts to anything 'chemically' but this has always worked really well
I used to use a shampoo called hexocal and it was fab you had to buy it from a vets. It was a tiny bottle that cost about 10 but you only needed a few drops and it lasted for ages. I used to get it from the local small vets as it was used for dogs as well.