The first specification of OpenCL (the Open Computing Language) has just been released by khronos. The specification can be downloaded here:
OpenCL is a general purpose data-parallel computing API, firstly initiated by apple and then standardized within Khronos. It provides a common hardware abstraction layer to program on data parallel architectures. It is very (very !) close to CUDA and supported by both NVIDIA and AMD, but also INTEL and IBM (Among others). This API don’t targets especially GPUs and we should also see CPUs implementations of it, but also maybe Cell implementation, and Larabee implementation (when it will be out) ? NVIDIA is likely to be one of the first to provide OpenCL implementation, since they already have their own now well-tried CUDA API to build on.
In addition to be cross platforms and cross-architectures, one interesting feature of OpenCL is that it have been designed to provide good interoperability with OpenGL, especially for data and textures sharing. It is meant to be the exact competitor to DX11 compute API.