Legacy school maths project
It was later hijacked and used to get my first grips with C++11's new futures and promises API. A simple project that can show multitasking in action pretty easily being implemented in a very simple way
For funzies I've made this into a benchmarking tool, alebit an extremely simplistic one. Run from CLI with -h to find out more (still ends up with graphical output due to switching that out being more work than it's worth given the scope of this "toy benchmark")