1) Project Serum is an upcoming decentralized ecosystem.
There a bunch of pieces, but here's the core:
1) a DEX
2) with sophisticated products
3) fully on-chain orderbook and matching engine
4) cross-chain compatible
and
5) actually fast and cheap
http://projectserum.com
http://sollet.io should now be fully-functional.
Mainnet works; testnet does too. No airdrops on those, though, but I can send some over from either for testing. Devnet is mostly deprecated.