NovaCoin
Hybrid energy-efficient digital currency
...Users hold the ECDSA private keys to their own money and transact directly with each other, with the help of the network to check for double-spending. They are also able to participate in the network protection using own money or computing power.
A hybrid Proof-of-Work & Proof-of-Stake scheme is used to provide a protection against double-spend.
The main project repository:
https://github.com/novacoin-project/novacoin