Proof-of-Stake Resources
CASPER
Casper is a security-deposit based economic consensus protocol that is set to eventually replace the proof-of-work incentive system currently in place for the Ethereum public network. The developers working on CASPER include Vitalik Buterin, Greg Meredith, and Vlad Zamfir.
My recommendation is to read these links in order to gain the best perspective on the thought behind CASPER proof-of-stake previously vs today:
"Slasher Ghost, and Other Developments in Proof of Stake" - Vitalik Buterin (10/3/14)
"Proof of Stake: How I Learned to Love Weak Subjectivity" - Vitalik Buterin (11/25/14)
"Introducing Casper “the Friendly Ghost” - Vlad Zamfir (8/1/2015)
"Vitalik Buterin Speaks About the Ethereum Foundation, Proof-of-Stake and More" - Coinjournal
Latest CASPER Specification/Implementations
Community Discussions of Interest
Tendermint
Tendermint is a proof-of-stake consensus protocol developed by Jae Kwon and Ethan Buchman. I have seen it implemented in Eris with an Ethereum VM backing.