XRB (RaiBlocks), it works well, the dev fixed any problem, human mining and still on little exchanges because it is difficult to implement (it doesn't have RPC calls like btc and many shit, but a different one, so you can't do copy and paste to have a wallet) and need a little PoW at each tx.

Bittrex was close to add it some time ago, but it never happened, at the moment 1000 sat is a great price to buy waiting other exchange.

Bitgrail and mercatox have them listed ATM.

PS. do some research about block lattice