Stats - Simple macOS system monitor in your menu bar.
TechTool Pro - Techtool Pro is a robust hardware diagnostic tool available for Macintosh.
iStat Menus - "An advanced Mac system monitor for your menubar."
DiskWarrior - DiskWarrior, the world's most advanced and highest rated hard disk, hard drive, and disk repair and data recovery tool for macOS, OS X and Mac OS X.
Stats Panel - OS & Utilities, Network & Admin, and System & Hardware
Disk Drill by Cleverfiles - Recover any type of deleted files in Windows & Mac including Office documents, messages, and media files quickly and easily.