StatsBar!

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).

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. »

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

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.

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.

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 »

This utility allows you to monitor a various system info in "always on top" floating window (overlay mode).Available features:- CPU Usage meter- Free RAM meter- CPU Frequency indicator- 2g/3g/Wi-Fi Traffic meter- Battery Level- Battery Temperature- Battery Current (mA)- Wi-Fi Signal Level- I/O Activity- Time/Date indicator- GSM Cell Signal Level (Samsung Galaxy S2 and Galaxy Note are not supported due to firmware bug!)- Number of fixed GPS satellites- SD Card free space- Most resource-intensive processes- Task Killer- Audio VU and visual equalizerAlso there is nice real time graph on background. Press Show Panel button to enable dashboard. You can drag floating window over the screen or double tap to open settings. See video and screen-shots for details.This application allows you to:- Easy diagnostic problem processes that consume resources excessively.- With Cool Tool you can monitor full-screen applications (games, video players, browsers).- Monitor your Internet traffic without closing the browser window.- Make your battery level always visible, with accurate percentage scale.- Monitor how good your CPU governor switches between CPU frequencies in idle/stressesDon't forget to try custom themes!Tips:Customize the appearance of the panel using color picker dialogs. Also you can adjust transparency and size of the panel in Sliders section.If you want the panel did not receive touch events, just select the "ICS hack" box. If you Android 4.0 user, you also may want to turn it on. Since in ICS the status bar is drawn on top of all. Even on top of Cool Tool:)Tons of gauges could drain your battery very quickly. Uncheck the features that you do not need. Btw, Cool Tool went to sleep and do not draw anything when your display turns off. It is for powersave.Warning! "Show most resource-intensive process" feature is pretty resource-intensive :) You should use it only if you really need it.If you want to hide Cool Tool panel in sp »

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.

Magican Lite is a free system monitor utility.

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. »