BitBar!

Rainmeter is a desktop customization platform. Through Rainmeter, you can enhance your Windows computer at home or work with skins: handy, compact applets which float freely on your desktop, much like Windows Sidebar gagdgets, or dashboard widgets for the Mac. Rainmeter skins provide you with useful information at a glance. Its easy to keep an eye on your system resources, like memory and battery power, or your online data streams, including email, RSS feeds, and weather forecasts. Many skins are even functional: they can record your notes and to-do lists, launch your favorite applications, and send your tweets to Twitter - all in a clean, unobtrusive interface that you can rearrange and customize to your liking. »

Conky is a free, light-weight system monitor for X, that displays any kind of information on your desktop. Originally a fork of Torsmo, Conkys torsmo-based code is BSD licensed. New code in Conky has been licensed under the GPL 3.0.

iStat is a series of system monitor apps. It's available in different versions: iStat Menus, iStat for iOS and iStat Server as a companion application to the iStat for iOS app that lets you remotely monitor your computers (free for Windows and Mac).

This is a port to OS X El Capitan 10.11 of MenuMeters by Alex Harper.

Mac OS X System Monitor and Process Explorer ToolatMonitor is the most advanced monitoring tool for Mac OS X that displays system activity in real-time. It is designed to allow quick assessment of computers state and it is powerful enough to make it a core utility in anyones system troubleshooting toolkit.atMonitor offers a better alternative to Apples own "Activity Monitor" and combines the best of the features of many other existing monitoring utilities. It also adds an extensive list of unique features of its own. In addition to the standard CPU, GPU, VRAM, FPS, Disk and Network info, atMonitor allows interaction with processes, setting up triggers, logging, purging RAM and much more. atMonitor can be displayed in the menu bar, as a stand alone floating window or in the dock icon. »

GeekTool is an application for Mac OS 10.6+. It lets you display various kinds of information on your desktop via 3 default plug-ins:

GKrellM is a single process stack of system monitors which supports applying themes to match its appearance to your window manager, Gtk, or any other theme.

MiniUsage displays various data like CPU usage, amount of network flow, battery status and process names which uses much CPU time in a menubar. It takes little space and can display various data, so it's suitable for notebook.

XRG is an open source system monitor for Mac OS X. XRG allows you to monitor CPU activity, memory usage, battery status, machine temperature, network activity, disk I/O, current weather, and stock market data. The goal of XRG is to be a functional system monitor that has a clean and flexible interface, so it doesn't intrude on the rest of your Mac experience.

Moo0 SystemMonitor is a smart free program that shows what is going on with your computer. The program uses a minimal amount of ressources, and you can learn from it where your computer "hangs". You can see your computer under many perspectives, and with the features you can diagnose your computer. You can have as many features as you want open and have them in the way that fits you best on the screen.It could be installed or also have a portable version.

SystemPal is becoming an ALL-IN-ONE app. It is the best tool to monitor your memory/CPU usage status, CPU temperature and fan speed, network speed. It also provides a tool kit including mini music player, screen shot utility, memory cleaner.

This little application displays system information on your desktop in a similar way to some other desktop information tools. Unlike others, this application looks like wallpaper but stays resident in memory and continues to update the display in real time. Uses very little memory and nearly zero cpu. Perfect for quick identification and walk-by monitoring of production or test server farms. Everything is customisable including language.

Nerdtool allows you to display many things directly on top of your desktop; weather reports, headline news, and system vitals are just a few uses.

Easily Monitor Your Mac's System Stats.