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.

ArtForz regularly doubles Bitcoins sent to the address 1PxuwbvLYZfBUt6iDGyvnDazcH9S7jNnMs, it is unknown how he does so.