To say that I bought a lot of yarn in 2008 is accurate. Most of my online yarn purchases were made through Knitting-Warehouse. Their website is simple, without the bells and whistles that others use to get our attention. They aren't, by any means, a closeout site -- KW has a variety of yarn in all price ranges. Though they say they cannot match dyelots 100% of the time, I haven't ever received a shipment from them with mis-matched lots.
In addition to yarn, they also have a great selection of needles, totes, books, and other accessories. While I've successfully steered clear from their book section, I have purchased bamboo needles and a tote from them. Both were great quality and low priced. I have never had any issues with orders/shipments from Knitting-Warehouse.
Shipping charges are very fair, and orders seem to ship within one business day. Overall, this website is tried and true (for me), and I will continue to shop with them again and again. 8 out of 10 (I'd like to see their selection expand).