SGX SDK for Linux
Software Guard Extensions (SGX) Software Development Kit for linux is designed to help application developers protect select code and data from disclosure or modification at the hardware layer using protected enclaves.
Project: https://software.intel.com/en-us/sgx
Code: https://github.com/intel/linux-sgx

Open Enclave SDK
Open Enclave SDK is an open source framework that allows developers to build Trusted Execution Environment (TEE) applications using a single enclaving abstraction. Developers can build applications once that run across multiple TEE architectures.
Project: https://openenclave.io/sdk/
Code: https://github.com/openenclave/openenclave

Enarx
Enarx provides a platform abstraction for Trusted Execution Environments (TEEs) enabling creating and running “private, fungible, serverless” applications.
Project: https://enarx.io/
Code: https://github.com/enarx