OpenCL miner
An OpenCL miner is a bitcoin miner that uses the OpenCL framework to perform the hashing computations. When used with a modern GPU, this can produce hash rates orders of magnitude higher than what can be achieved with a CPU.
Overview
OpenCL is a computing framework which allows programmers to write code which works across a variety of hardware, both CPUs and GPUs included. OpenCL was originally developed by Apple.
The use of an OpenCL miner allows one to hash using a GPU (Graphics Processing Unit), which has a vastly larger ability to perform algorithmic calculations than an CPU (Central Processing Unit).
Requirements
Hashing on a GPU requires an OpenCL compatible (ATI/AMD) or CUDA compatible (NVIDIA) graphics card.
The python Poclbm open source OpenCL bitcoin miner was created by m0mchil[1] based on the open source CUDA client originally released by puddinpop[2].
Subsequently the java DiabloMiner based on m0mchil's was created by Diablo-D3[3].
Puddinpop has also released a C++ OpenCL client capable of being compiled into the GUI client or daemon, or as a stand alone RPC miner[4].
Another miner was released by Jedi95 & CFSworks released Phoenix miner[5] which can work with memory underclocked to 300MHz and scans whole 232 nonce space.
See the Mining Hardware Comparison page for detailed statistics on the hash rates that can be achieved with various hardware.