Orbitz used to be unique in constantly having 10 or 15% off codes available, but now it seems like hotels.com (and possibly other Expedia properties) have them too. I’ve generally used Orbitz but might now consider hotels.com since the free night seems more appealing than Orbitz’s perks (I’m already on track to max out status on Orbitz, and hardly any hotel participates in VIP perks). For Europe I’ve often used booking.com, whose status (which gives 10% or 15% off at many hotels) is based on a low # of lifetime stays (and currently doesn’t expire on an annual basis).