Intel® SGX
Intel® Software Guard Extensions (Intel® SGX) Software Development Kit 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
Red Hat 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