I tend to favor HauteLook.com and have actually bought a few things off this site. HauteLook features men's and children's clothing in addition to women's and you can preview upcoming sales and see what is releasing soon.
Ideeli is another membership site that tends to have a lot of purses, jewelry and other accessories. They also do giveaways of great stuff like Prada bags or high-end jewelry which you can enter regularly (which I do, but never win). This site has a "1st row membership" that you can pay for in order to get early access. I find the best stuff is usually gone by the time I get to it, so it would probably be worthwhile to sign up for a paid membership.
Then there is GiltGroupe which has the most men's clothing of the three of these and usually releases a few sales at the same time so there is a lot to choose from. This site also releases sales that feature several designers at once. I've found a lot on here that I love, but have not bought yet.