iStat Menus - "An advanced Mac system monitor for your menubar."
GPU-Z - GPU-Z is a lightweight system utility designed to provide vital information about your video card...
Stats - Simple macOS system monitor in your menu bar.
CPU-Z - CPU-Z is a freeware that gathers information on some of the main devices of your system : Processor name and number, codename, process, package, cache levels.
MenuBar Stats - Advanced Mac system monitor right from the menu bar
Speccy - Speccy - find the details of your computer's specs. Great for spotting issues or finding compatible upgrades. Download the latest version free.