I dont know how I ever lived without...
My yarn swift and ball winder, recently purchased from ebay:
I seriously don't know how I did without these, they're able to convert hanks into flat bottom balls (*smerk*) of yarn in around 30 seconds! As long as your hank isn't tangled.... i had one that was which i couldn't wind and took me three hours to unravel.
And then once you're done you have the yarn with a flat bottom and top so when you're knitting with it, it doesn't roll around everywhere! I forget how much I paid for them both exactly but it was well over $100 and i think closer to $130 but it was totally worth it and I'd recommend that anyone who doesn't have them invest in them. Its like a tonne of weight being lifted from your shoulders knowing that you can convert your hanks into balls with very little fuss!
one word: Awesome!