Some cartridge bearing headsets like the Velo Orange Gran Cru and some FSA models offer a split "crown race" (actually its just a bearing support, not really a race) which greatly eases installation and removal. I've got a VO on one bike and it was a joy to install.

Others, also with cartridge bearings like Cane Creak and Chris King, provide a continuous "crown race" but it's not needed and the user would be better off if they split them.