A very influential anthology series edited by Harlan Ellison. The first volume, Dangerous Visions, was published in 1967 and was an anthology of stories with themes and/or content which was "dangerous" by contemporary sf standards. It included major introductions by Ellison, along probably worth the price of the book. It was followed in 1972 by Again, Dangerous Visions with Last Dangerous Visions a projected third volume.
LDV was massive — it may well have had more than 100 stories — and along with Ellison's enormous introductions may have been unpublishable. In any event, it was never published and became a bone of contention between Ellison and nearly everyone else. See Last Dangerous Visions for more.