A while ago, I read on someones weblog (I can't find it anymore, but if it's yours, just let me know, so I can put in the link) about unraveling old sweater to use the yarn. I still had some old sweaters lying around, all in heavy cotton, and I decided to try to frog one. I found a nice tutorial here. I didn't 'wash' the yarn to get it straight, but I might do so before using it.
And this is the result! Great soft red balls of cotton yarn to use. I want to make a bolero or a sleeves top from it to wear in winter.