Conky
Conky is a free, light-weight system monitor for X, that displays any kind of information on your desktop.
14 Alternatives To Conky
Bginfo
This fully-configurable program automatically generates desktop backgrounds that include important information about the system.
BitBar
Put anything in your Mac OS X menu bar
Core Temp
Nov 4, 2017 - Core Temp is a compact, no fuss, small footprint, yet powerful program to monitor processor temperature and other vital information.
MenuMeters for El Capitan
MenuMeters for OS X El Capitan 10.11 (and later)
Moo0 System Monitor
Lets you keep your eye on system resource usages of your PC.
Nerdtool
NerdTool is a GeekTool-like application that displays information directly on your desktop.
Open Hardware Monitor
Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.
Rainmeter
Rainmeter is a desktop customization platform.
Sidebar Diagnostics
SidebarDiagnostics - A simple sidebar for Windows desktop that displays hardware diagnostic information.
XWidget
XWidget is a free desktop customization platform.
glances
Glances is a cross-platform system monitoring tool written in Python. Written in Python, Glances will run on almost any plaftorm : GNU/Linux, FreeBSD, OS X and Windows.
htop
htop - an interactive process viewer for Unix. This is htop, an interactive process viewer for Unix systems. It is a text-mode application (for console or X terminals) and requires ncurses.
iStat Menus
“An advanced Mac system monitor for your menubar.”
netdata
Real-time performance monitoring, done right