Knit Picks is an online store with "Knitting Yarn, Books, Patterns, Needles & Accessories". They are my favorite needle maker, and have a full line of wonderful yarn. They are headquartered here in Vancouver, WA.
Ravelry is a wonderful organizational site for knitters and crocheters that has social networking built in too. They are opening it up slowly, so if you aren't in yet, get your name on the list!
...how to obtain beautiful results when mixing and matching yarns; topics include how to deal with different gauges, how to use colors and how to fake a multi-yarn look.