The DirectX graphics API created by Microsoft is used by nearly all PC game developers. The one big exception is id Software. Its main programmer John Carmack has used the OpenGL graphics API to help program all of id's 3D graphics engines including id Tech 5 which is the basis for id's upcoming shooter Rage.
While Carmack has been critical of DirectX in the past that has now changed. In an article at bit-tech.net (based on an interview Carmack gave to the Custom PC print mag) he states, "I actually think that Direct3D is a rather better API today." He added that while DirectX has improved over the years, OpenGL "has been held back by compatibility concerns."
So does that mean that Carmack will switch to DirectX? Maybe not. He states, "OpenGL still works fine and we wouldn't get any huge benefits by making the switch, so I can't work up much enthusiasm for cleaning it out of our codebase."