Foundry

Foundry is a Rust-based smart contract development toolkit that replaced Hardhat for a lot of Solidity developers who wanted faster testing and native Solidity scripting. Write your tests in Solidity instead of JavaScript, and they run orders of magnitude faster. Apache 2.0. Four CLI tools: forge (build/test), cast (interact with contracts), anvil (local testnet), and chisel (Solidity REPL). The testing framework supports fuzz testing, invariant testing, and gas snapshots natively.

Fully free. No paid tier, no hosted version. Install the CLI and go. The speed difference over Hardhat is not marginal; it's dramatic. Test suites that took minutes run in seconds. The Solidity-native testing means no context-switching between Solidity and JavaScript.

Solo through large teams: free, no restrictions. The tooling is mature enough for production use at every scale.

The catch: Foundry's Solidity-only testing means you lose access to Hardhat's JavaScript plugin ecosystem. If you need complex deployment scripts with off-chain logic, Hardhat's JavaScript flexibility is easier. The learning curve from Hardhat to Foundry is real: different testing patterns, different project structure. And if you're not in the EVM ecosystem, this is irrelevant.