Addressing your questions specifically:
- Decentralized digital currency = Bitcoin: http://bit.ly/1PovAAT
- Federated digital currency = Ripple: http://bit.ly/1O4TzRx
- Centralized digital currency eCurrency Mint = http://bit.ly/1o2vKo9
My assessment is that while all have their applications (other than currency), I see the centralized model winning out -- similar anonymity benefits as Bitcoin -- as currencies are the domain of sovereign nations that will fight at all costs to prevent any disruption to that. Thanks for asking.