I'm sure by now many people have seen the ads for websites like Ideeli or HauteLook. Essentially, they are like online outlet malls for designers. You sign up for access to each day's sales that typically last 1-4 days (depending on the designer), and you get last season's stuff at a greatly discounted rate.
Bluefly.com - One website that your don't need a membership for. Their items for sale are not time sensitive and they have a much broader range of selection, but unless they are clearance their prices aren't as good as the others.
HauteLook - HauteLook has new sales every day and features a bigger variety of types of products than some of the other websites. They have clothes for men, women, children, and babies but also accessories, home products, and food. Items go FAST so if you are looking at a top designer, these sales are essentially sample sales meaning there are only a very limited number of items. If you don't sign in when the sale begins, you may not get your item. Also, if you join and get your friends to join you can get credits towards your purchases.
Beyond the Rack - (this merged with The Top Secret) mainly features fashion. Many different designers, but also many I had never heard of before. Good or bad thing? I guess it depends on how label conscious you are and your style.
ideeli - I think ideeli is probably the best/my favorite of these websites as far as discounts and designers goes. I have only seen fashion on here and most of the designers are all big name ones with some MAJOR discounts. The one downside is, they offer a paid membership on top of the free one. You get access to sales an hour earlier. For a website like HauteLook, this makes a major difference (although they don't have this option), I'm not sure if it makes a huge difference here. I guess it depends on the designer and item since they typically have more inventory. This website (like the other two) also feature the friend incentive program.