http://www.ebay.com
http://www.shopbop.com
http://www.LoopNYC.com
http://www.netaporter.com
http://www.eLuxury.com
http://www.adasa.com
http://www.officeholdings.co.uk
Also, the NeimanMarcus and Nordstrom websites have awesome mark downs off designer clothing during their sales.
Don't even bother with Bluefly.com... their selection consists of wack overpriced clothing from 5 years ago.
If you like online window shopping, also check out Nanette Lepore and Charlotte Ronson.