I am aware of "semi-flex" searches, many US portals (aa.com etc) offer these in which your departure is flexible but not both departure and return. Basically, you are half-blind. They'll tell you how low 15-16 nights ticket will get, but if you might have saved hundreds by staying 14 nights or 17 nights, they won't mention it.

AFAIK, only zuji.sg offers a meaningful flexible search which is flexible at both ends. For those with only a few days of flexibility, Kayak has a +/- 3 days option at both ends; in my experience it has not been a reliable locator of the cheapest options, but no harm in trying (registration is required to use this feature).