It's a little bit of a luxury, but a very useful thing to have nonetheless: a yarn butler. I finally treated myself to this one from ChiaoGoo during one of my January shopping sprees at Purlescence.
It's already come in handy for my jumper knitting. It stops the yarn from rolling around, keeps it in one place, and I find it especially useful when knitting with two colours. I keep one ball on the yarn butler and the other somwhere nearby so they don't get too tangled.
This yarn butler is surprisingly heavy, but it is very sturdy and I like it. When I tug on the yarn, the ball bearings inside the base make a rumbling sound that I mistook for distant thunder at the beginning. It runs very smoothly and I look forward to using it regularly.
What's your latest knitting helper?