This is a great pattern for leftovers of not too thin (or too thick) yarn. I stumbled upon this ball a few years back, but never found a pattern for it, so I decided to find out how it was made myself, from just a photo. Took me a few nights, but then I had it.

I just finished the English translation of this pattern, and it is now available as a free Ravelry-download.

Can be knitted in a few hours. Great project for children who just stated learning how to knit as well, although they need a bit of help assembling it. Can also be crocheted, with a little imagination.

Update december 2012 :
Raveller Cheryl Brunette has made a great instruction-video on how to assemble the ball on Youtube. I thought a picture said more then a thousand words, but a video definitely says more then a thousand pictures !
Find it here : http://www.youtube.com/watch?v=pZA_kgR9bGc&feature=plcp