MiniUsage!

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. »

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.

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:

XWidget is a free desktop customization platform. It’s light, handy with powerful visual widget editor and very subtle animations. The Android page is found here: http://android.xwidget.com/

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.

The BitBar app lets you put the output from any script or program right in your Mac OS X menu bar. And it's completely free. An impressive number of plugins have already been contributed by a wide range of developers just like you, and this site makes it easy to find them.

Monitor numerous OS X statistics including: CPU activity System load over time Network bandwidth and activity Memory activity and usage Battery & wireless signal strength Disk usage Current time and date Compact, clever & fully customizable user interface Presents information both graphically and with text Beautiful on Retina display Fits in perfectly with the new system appearances Save & load custom appearances Minimizes impact on system performance View data on the desktop, in the menu bar or Dock Show & hide windows using system hotkeys »

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.

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.

SysMetrix is a skinnable clock and metering application. Its purpose is to provide system metrics in a variety of interesting, useful, and cool ways. It can monitor and report on the hundreds of statistics

Professional System Monitoring Utility designed for Mac.

Stater is an application that displays system parameters (i.e. CPU, memory, and drive space usage charts), directly in the dock icon. Besides four icon views, the dock menu contains detailed information about CPU, memory, drive space usage, and top processes. Stater has no windows or menubar icons. All control, including settings, takes place via the dock icon and dock menu.

Translucent provides detailed real-time information about what is going on inside your Mac. At any given time Translucent displays how much each one of your processors is being utilized, detailed information about how your memory is being used, how much data is being sent and received on your network interfaces and how much is being read and written to your hard disks. It displays itself as small addition to your status bar with tooltip-style detail popups; displaying more detailed information on any of the sensors in the status bar. Translucent also displays its sensor information in windows for larger and more detailed information viewing. »