I know NXT and BitShares have their own PoS implementations but most PoS coins currently out there are based on Peercoin aren't they? And Peercoin was the first implementation of PoS.
I think a lot of coins have been forking Blackcoin for PoS for the last while. And I'm not sure but I'd assume that Blackcoin is a modified fork of older Peercoin/PPCoin.
It's a fork of Novacoin, which is a fork of peercoin. They changed from that to "POS 2.0". CfB reviewed it and said it was now almost the same as Nxt.
Not sure where that fits.