I've been working on a tool for brute-forcing Bitcoin private keys. The main purpose of this tool is to contribute to the effort of solving the Bitcoin puzzle transactions: https://blockchain.info/tx/08389f34c98c606322740c0be6a7125d9860bb8d5cb182c02f98461e5fa6cd15

The performance is good, but can likely be improved. On my hardware (GeForce GT 640) it gets 9.4 million keys per second compressed, 7.3 million uncompressed.