Best tools for testing various hardware components?

I like building useful PCs out of older or used components.

What are the best software tools for stress testing used/refurbished components such as the CPU, RAM, SSD/HDD, and GPU to make sure they are fully functional and stable?

I know for memory, Memtest86 is a good one. Is there a useful memory testing tool that can run in the OS? There are several GPU benchmarks out there but I'm not sure what the best is for raw stress testing for stability.

What about SSD/HDD stability tests?