Atmonitor!

Process Hacker is a feature-packed tool for manipulating processes and services on your computer. A free and open source process viewer with powerful process termination and memory searching/editing capabilities.

System Explorer is an enhanced Taskmanager that provides detailed information about running processes, startup programs, IE add-ons (BHOs), drivers, services network connections, system resources and more. It also keeps track of a history of processes that were started, and allows you to create file and registry snapshots that can be compared to analyze system changes in detail. System Explorer integrates online lookups into the right click menu for any process, driver or service, allowing you to retrieve additional information (and virus checking) from online sources including Google. »

Windows Task Manager is a task manager application included with Microsoft Windows NT family of operating systems that provides detailed information about computer performance and running applications, processes and CPU usage, commit charge and memory information, network activity and statistics, logged-in users, and system services. The Task Manager can also be used to set process priorities, processor affinity, forcibly terminate processes, and shut down, restart, hibernate or log off from Windows. »

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).

Glances is a CLI curses based monitoring tool for GNU/Linux and BSD OS. Glances uses the PsUtil library to get information from your system. It is developed in Python.

System Monitor is a tool to manage running processes and monitor system resources.

Easy to use application for monitoring and controlling running processes.

Resource Monitor is a system application in Microsoft Windows operating systems. It is used to view information about the use of hardware (CPU, memory, disk, and network) and software (file handles and modules) resources in real time. Resource Monitor is available in Windows Vista and onwards only (In Windows Vista, it is known as Reliability and Performance Monitor). Resource Monitor can be launched by executing resmon.exe (perfmon.exe in Windows Vista). It was also on Windows 95, 95 OSR, 95 OSR2, 98, 98SE, ME, and NT. »

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 »

KillSwitch is an advanced system monitoring tool that is apart of Comodo Cleaning Essentials (CCE) it allows users to identify, monitor and stop potentially unsafe processes that are running on endpoint systems. Uniquely for a product of its type, KillSwitch leverages Comodo's huge whitelist database to isolate untrusted processes with an extremely high degree of accuracy – improving IT operational efficiency by reducing false positives and cutting the amount of time spent troubleshooting an endpoint machine. »