All asset platfroms have it like Etehreum, Counterparty, NXT, ETC, waves, bitshares...  but you can only trade asset that was issued on their platform. 
I think most popular decentralized exchange where you can trade different coins is the one that used to be called bitsqare and was latter renamed to https://bisq.network/  Sadly had only 3 BTC volume today    IF we will not start using decentralized exchanges then they will never get alive.