Gilt - Good selection but price usually not the lowest unless you get lucky. If I see something I like on Gilt, I normally google and see if I can find it cheaper elsewhere before buying. Shipping is always $6 at the cheapest and free for other items that you order within an hour of the one you paid shipping for. You can collect Gilt points to use towards free shipping or discounts on items. A lot of items are only eligible for Gilt credit when you make a return with free return shipping. Sometimes you can return for refund to your credit card but you pay return shipping. Always read item description carefully being purchasing. Absolute best referral credit per friend you refer - $30 to you if they buy something. Sales start at 9 am PST unless you use points to shop early. Noir members (they upgrade 1-2% of members who shop a lot, like yours truly lol, to Noir status) can view sales an hour before sales start but cannot purchase. There are also sales that start at 6 pm PST daily, as well as Sunday night deals.
Ruelala - Not the best selection and prices are not that great either. I only shop here once in a blue moon now. Free shipping for 30 days if you pay $10 once. I think you can also pay for a year of free shipping but I've never done that. $10 credit to you per referral. Sometimes they give you free shipping for 7 days if your friends who you've referred before (and you've already gotten referral credit for) buys something again. Return for either credit and get free return shipping, or to your credit card and pay return shipping. Sales start at 8 am PST. Sunday nights have "Sunday Night Steals" that start at 5 pm PST and have good prices, but items tend to sell out super fast.
Hautelook - Now owned by Nordstrom so you can return items to Nordstrom Rack. Free shipping if you spend over a certain amount. $10 referral credit per friend who makes a purchase. Sale starts at 8 am PST. They are located in California, so this is the only site that I have to pay sales tax on items I purchase since I live in CA. Good selection and prices, but I don't really shop here often anymore because of taxes. The offset though, is that when they ship something out, it is usually from LA area and I get it the next day.
ideeli - Best prices of all these sites IMO. Many times, I've found that same exact items will sell for half the price (or even less) on this site than on Ruelala or Gilt. Pay $10 shipping one time and get it free for the next 30 days. Return for either money back (but pay return shipping) or credit (free return shipping). Has online chat customer service which is very convenient. Referral credit $10 per friend that makes a purchase. Sales start at 9 am but you can view sales early, just cannot buy until 9 am. First row members can start buying at 8 am. You can pay to have first row access, but they normally just give it to you for free for a few months at a time. I've never paid for it but always seem to get it.
Beyond the Rack