Recently there seems to be a slew of new websites popping up that offer high fashion at low prices.  So I have two great finds for this post (and I have made purchases on both sites):

• Ideeli - this is an online auction where they sell a deeply discounted item daily.  They will send you an email notice and you have a set amount of time (or until they run out) to buy that item (this is a lot like SteepAndCheap which does the same thing with sports equipment and outdoor gear).  You can pay an annual membership and shop the sales an hour early (which I am waiting to do until I find something truly great).  I have bought a watch (it was a $600 watch, and I paid $135 for it) and it arrived promptly (note that when I made this purchase you had to have the same shipping and billing address).  They have big ticket items like Prada bags, Dior sunglasses, etc.  It is pretty hit and miss, and sometimes the good things disappear quickly, but every once and while you can find a really great bargain.   However, everything is final sale so make your purchases carefully. (BTW, I have no idea how they source their supply--it could be last season's or last year's items--or maybe it is just a special deal)
• RueLaLa -  this site is like an online sample sale.  Once every couple of days or so they sell a few items from one brand in a little boutique.  I have seen clothes from BCBG Max Azaria, outwear from Ugg Australia, and lingerie from Cosabella as some sample boutiques.  I have also seen them list high end household items like cooking wear, and I purchased some sheets.  They have a 30 day return policy for all items and I was impressed with the high quality packaging (and prompt arrival of my purchase).  The only thing is, if you don't buy quickly you are out of luck, things sell out really fast.  (I also don't know how they stock their merchandise, but it also tends to be a pretty good deal--usually around 50% off).

PS: There is also a site called Gilt Groupe that was founded by at least one graduate from Harvard Business School (if I recall correctly) but I have been much less impressed with their site and found it hard to use and not the best experience, so I am not listing it here.