Ethereum/Ethereum Classic: Smart Contracts
Ripple/Stellar Lumens: Payment conversion between currencies
Waves: Smart contracts, exchange
Golem: Cloud computing
Steem: Social network
Gnosis: Prediction market (Running on Ethereum)
Augur: Prediction market (Running on Ethereum)
Sia: Cloud storage
BitShares: Exchange, Price-stable pegged smart assets, prediction markets
MaidSafe: Cloud storage, DNS, internet infrastructure/website hosting

That's a quick pass through some of them...