Linux
The Linux kernel itself has a module, specifically Linux Security Module(LSM), called Security-Enhanced Linux(SELinux) that has enjoyed official integration status with the mainline Linux kernel since 2003. Despite Linus Torvalds, the kernels gatekeeper, initial objections the NSA eventually got him to accept their proposal to integrate SELinux. However, there have been specialized distributions and projects which attempt to make Linux more secure for particular scenarios.
Read more about this topic: Security-focused Operating System