ModCloth is my favorite. It has a large, expanded size range, and some amazingly unusual designs. My one complaint is that many of the clothes are in polyester or rayon; I prefer cotton. That said, the tee shirts I've bought there are my favorites and I never see someone wearing them aside from me!
Macy's is a wonderful online site for basics. So much easier to order than to trek down to the store, especially if I want a particular size and don't have a lot of time to look. I can order it from Macy's and it takes less than a week to arrive. Bonus: I can return items at the store. The best of both worlds!