I sometimes find myself admiring the intricately embroidered and printed curtains at Anthropologie, but when it comes down to it, I stick to basics for my windows. Standard-issue PVC blinds are pretty awful, but white curtains and bamboo or reed shades go with everything and are classic and affordable.
I prefer to have the windows themselves and the incoming sunlight stand out more than my choice of fabric. (If it were practical, I'd just leave my windows bare.) For rooms that don't get a lot of light, the white is bright on its own. If you want something more adjustable, blinds and roman shades made of natural materials add texture and a little color without competing with their surroundings.