Peercoin was the first and it uses minting which is based on coin age. NXT has transparent forging which uses a random process to determine who forges the next block ahead of time although it hasn't been fully implemented yet. BitShares uses delegated proof of stake (DPoS) which involves a system of 101 delegates who process transactions and are voted into their positions.
This thread was created in April 2014 and back then, these were the three different PoS systems available.
Since then, NEM has launched which introduced proof of importance (PoI). Reddcoin seems to have invented yet another variant of PoS called proof of stake velocity almost a year ago:
http://www.reddit.com/r/reddCoin/comments/249dnl/major_announcement_reddcoin_to_implement_new/Are there any others that I might have missed out? I think ReserveShare had something similar to PoS called proof of reserve which worked in a similar way to a savings account but that one turned out to be a scam.
