Bing through Yahoo BOSS API (which isn't free anymore) is the main source (and Yandex is used instead of Bing for certain countries in Europe&Asia). All other sources are smaller sources that augment the main search results. DDG is at the moment a meta search engine.
There was a recent search engine startup with its own large scale crawler, Blekko. Though, recently IBM bought it for their Watson AI, as a knowledge base.
