Luxury online merchants have the largest collection of designer goodies under any one virtual roof. From Prada to Dolce & Gabbana, from Brooks Brothers to Nicole Miller, it’s all here: handbags, shirts, dresses, shoes, jewelry, home furnishings, the works. If you’re overwhelmed by the possibilities and just want to make the designer-label lover very, very happy, just put some money into an online gift certificate and set ‘em loose.