Process Monitor!

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. »

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.

API Monitor is a Windows program that lets you monitor and control API calls made by applications and services. Its a powerful tool for seeing how applications and services work or for tracking down problems that you have in your own applications.

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

This tool controls programs, disk, CPU. Replace task manager, tweak and tune up XP or Vista. Speed up windows startup. Get rid of spyware and viruses.

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. »

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. »

SpyStudio is the best product to trace user-mode API calls on Windows. SpyStudio is an Application tracer which shows and interprets calls, displaying the results in a structured way which is easy for any IT professional to understand. SpyStudio can understand the resources that an application uses, track down errors, detect malware and create application layers for virtualization.SpyStudio has a trace comparison tool which is useful for identifying problems in virtualized environments. You can obtain two samples: the native version of the application and the virtual version which has the problems. Using the 'Compare traces' feature, you can see what is missing in the virtual environment. SpyStudio is the user-mode SysInternals Process Monitor's (aka Procmon) complement. Looking for application errors with kernel-mode traces is tedious, and it is very difficult to see the final outcome of a user-mode call. With kernel-mode tools, you get a lot of noise that the application does not see, since a single user-mode call generates lots of kernel-mode events that are not important from the application's perspective. Most application errors are generated by failed user-mode calls which expect a different state of some resources: registry keys and values, files, pipes, services and printers.SpyStudio is also able to load and interpret Procmon logs. »

KSysGuard, also known as KDE System Guard and KDE System Monitor, is designed to make simple process control available to a user without needing to do any special setup - the defaults are usually perfectly adequate.

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 »

Linux process explorer, behaves the same as the windows process explorer from Mark Russinovich.

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. »

LXTask is the standard task manager and system monitor of LXDE. It starts via Ctrl+Alt+Del and is extremely lightweight.

vtop is a graphical command-line tool that uses unicode braille to chart CPU and memory usage. It can be easily extended.