ArtForz

ArtForz is a regular on #bitcoin-dev. He was the first person to mine with GPUs, using private mining code that he wrote himself. He continues to hold a high percentage of network computation power (~25%), and he claims that his mining software is still more efficient than all other GPU miners. He uses a heavily-modified version of Bitcoin; notably, he requires less transaction fees and he doesn't perform IsStandard checks.

ArtForz has a very good understanding of Bitcoin. He reported serious bugs in Bitcoin's handling of certain script opcodes.