While Ethereum is the most popular platform for writing contracts, it is not the only one. The following are some of the other platforms used for writing smart contracts
Bitcoins: Bitcoin uses Script that has limited capabilities when processing documents. The Scripts are specifically designed to process Bitcoin transactions.
Hyperledger Fabric: In Fabric, Chaincode is programmatic code deployed on the network, where it is executed and validated by chain validators together during the consensus process.
NXT: It's a public blockchain platform that contains a limited selection of templates for smart contracts. You have to use what is given, you can’t write your own code.
Side Chains: Side chains enhances Blockchains performance and privacy protections. They also add capabilities like smart contracts, secure handles, and real-world property registry.