Yesterday, February 16th, the Khronos Group released Vulkan, which is a graphics and compute API for the GPU. To some extend it is the successor of OpenGL, though Vulkan will not replace OpenGL. The Vulkan API is designed to eliminate most of the complexity and overhead that is required to implement the OpenGL API. Furthermore, Vulkan has much better support for multi-threading, which allows you to distribute your rendering calls over multiple CPU cores. If you care a lot about performance, you should definitely check it out. Otherwise, stick to OpenGL, because Vulkan is (supposedly) much harder to use.
Update: I have a Fermi GPU, which is Vulkan capable, but it seems that NVIDIA will not be providing Vulkan drivers for Fermi GPUs. This means that I won’t be trying to use Vulkan for the time being.