Ethereum, the well-known global blockchain platform was the first to introduce smart contracts to a more wide-spread crypto community. Ethereum is still the most advanced platform for coding and processing of smart contracts. This open-source platform has one of the largest networks of developers available, and due to this, it can keep up with the continually changing environment in the blockchain industry.
Using a hybrid of Proof-of-Work and Proof-of-Stake model, Aeternity offers a method for powering so-called Turing-complete smart contracts that are capable of being executed off-chain. Thereby they deliver both privacy and security.
Cardano is a decentralised blockchain and cryptocurrency project. Like many crypto projects, Cardano is open source. The Cardano platform is working towards implementing smart contract functionality with the Goguen update this year. This should bring their smart contracts a step further to 'smarter contracts'.
Qtum is an open-sourced blockchain application platform, where security and flexibility are two of the most essential components. The Qtum team has worked intensively to assure that smart contracts can be executed safely, making the platform perfect for businesses and their enterprise clients. Qtum uses Proof-of-Stake and a Decentralized Governance Protocol.
Stellar, unlike many crypto coins, was created by developers for developers. That means that it is capable of handling extremely complex smart contracts. For simple smart contracts, Stellar offers a clean, easy-to-use alternative for developers that want to build smart contracts delivering greater efficiency.
Waves is an open blockchain project, strongly focusing on dApps and using Web 3.0 technology. To keep their smart contract project simple, Waves offers many online courses, and other methods of support for developers that may want to work with Waves. Like many smart contract projects, Waves uses Proof-of-Stake.