MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)
GPU-Z - GPU-Z is a lightweight system utility designed to provide vital information about your video card...
iStat Menus - "An advanced Mac system monitor for your menubar."
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.
Stats - Simple macOS system monitor in your 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.