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 - Simple macOS system monitor in your menu bar.

Data Rescue - When Apple's system recovery fails, recover any and all files with Data Rescue 3. This data rescue software will even work if your Mac's hard drive fails to mount.

iStat Menus - "An advanced Mac system monitor for your menubar."

DriveDx - DriveDx is advanced drive health diagnostic and monitoring utility.

MenuMeters for El Capitan - MenuMeters for OS X El Capitan 10.11 (and later)