Reason why most things are old is because they were well built. Basically anything that puts CPU to 100% and GPU to 100% will stress test in terms of heat. Things like SuperPI on multiple cores can stress out a CPU, furmark will certainly stress your GPU.
My personal favorite is actually a bit strange but helps stress the computer closer to actual stresses it'll see in games (without overheating issues from furmark). Basically download the Unreal Engine 4 demos and Bullet Physics Engine, compile a simple Bullet Physics Engine demo with as many threads as your computer has, each with a different world and solver (SI, NNGC, Dantzig, GS, etc) and a string of a dozen 6dof constraint boxes between two walls, with gravity rotating in a...