America's Yarn Store® offers the most comprehensive selection of yarns, needles, books, patterns and supplies for knitters, crocheters, weavers & more!
http://www.yarn.com/

Yarn is a long continuous length of interlocked fibres, suitable for use in the production of textiles, sewing, crocheting, knitting, weaving, embroidery and ropemaking.
http://en.wikipedia.org/wiki/Yarn

YarnMarket has an extensive array of yarn, knitting patterns, books, needles, projects, kits, bags, tools and accessories to meet the diverse, creative needs of the knitting ...
http://www.yarnmarket.com/

Do you love yarn? Lion Brand Yarn has thousands of free knitting and crochet patterns plus lessons, clubs and more for those who knit, crochet and craft with Yarn.
http://www.lionbrand.com/

The largest selection of designer knitting yarns on the internet at discount prices. Brand names include Noro, Debbie Bliss, Prism, Koigu, Berroco and Trendsetter.
http://www.gotyarn.com/