There is currently an actively developed alternative cryptocurrency called Monero, that uses the 'Cryptonote Protocol', and has a completely different codebase

There is currently an actively developed alternative cryptocurrency called Monero, that uses the 'Cryptonote Protocol', and has a completely different codebase from Bitcoin. It uses Ring Signatures and a few other cryptographic tricks to make the currency provably unlinkable and untraceable. In 6 months, the network will hard fork to support what is being called "RingCT", which will hide the amount sent in a transaction, in addition to the sender and the receiver that are hidden at the moment. If you want to read some more about it, ask all the questions you like on /r/monero, they'll be happy to get as technical as you want.