Used Resources

File

See: section about File in use.

Disk space usage
Seeing how much space is free

See the section about Checking for amount of free disk space.

Finding out what is using up disk space

See the section about Finding out how much disk space is used.

Dealing with having too little available disk space where it is needed
See the section about Dealing with having too little available disk space where it is needed.
Names
Names are discussed at: device addresses.
[#bsyrgsry]: Registry in Microsoft Windows

If it is known/suspect ahead of time that a program may modify the registry, use NirSoft's RegFromApp to either monitor an already-running process, or to start a process. (Note: This hasn't been heavily tested. Will this work even if the program spawns a separate program like REG.EXE?)

[#busydisk]: Disk responsiveness/time

This section is about a disk being used heavily. For details about what file is open (which may cause the disk to be considered to be “in use”), see the section about a busy file.

See: busy disk.

[#cpuusage]: CPU usage
See: CPU usage
[#hwcpused]: Finding what is using the CPU
See: CPU usage: section about finding what is using the CPU
Verifying that the CPU is being heavily used
For verifying that the CPU is being heavily used in Microsoft Windows, information is provided using both text mode programs and graphical programs. There's also a discussion for Unix, including misconceptions about load.
Controlling CPU usage
Controlling CPU usage
Handling CPU idleness
Handling Idle CPUs
Emulating a CPU
CPU emulation

Controlling CPU speed
See: CPU slowdown and overclocking.

Note: for other details about CPUs, see Hardware section: Central Processing Unit

Memory usage
See: Low memory
Network
bandwidth/utilization/speed
See: network troubleshooting, speed testing
Transport Layer Port Usage
[#whotcp]: TCP and UDP ports

With both Unix and modern versions of Microsoft Windows, using “ netstat -na ” can help to at least verify that a port is being listened to. However, what can be even more useful is know which program is listening to that port. The network stack should be able to keep track of this information, because the network stack needs to know what program needs to receive any incoming traffic.

Windows
Command line options

In supported versions (some further testing may be done to clarify which ones), one may use: netstat -nab | more

The “b” in that command line provides details about what process is associated with a connection. That can be notably more informative, but it also may require a new-enough version of Windows and it may require that UAC is not blocking that information. (If UAC is blocking the information, ways to deal with UAC may help: perhaps more success can be obtained by starting a command line as an “administrator” from UAC's perspective.)

Graphical interface

With even newer versions of Windows, there may be a graphical approach available. (This works in Windows 7.) Go to the Resource Monitor (reachable from the “Networking” tab of the Task Manager), and then view the “Network” tab.

BSD
OpenBSD

To check what is listening to UDP port 123, use:

fstat | grep udp | grep :123
FreeBSD

FreeBSD FAQ 13.3 (extra named port used by BIND) compliments users of sockstat: “Congratulations, by the way. It is good practice to read your” sockstat “output and notice odd things!”

(Admittedly, this guide may be better once it provides information on how to use that software...) FreeBSD Manual page for the sockstat command

Network usage

Some options exist to see what is using up network bandwidth.

One option may be to see networking sniffers, which are capable of monitoring traffic.

General

Some software reports on various different system usage, such as CPU usage and memory usage and disk usage and perhaps some other type of general I/O usage, such as interrupt usage. Such software may often have multiple reports, and so viewing another report may provide more specific areas (such as just showing details about what is using up a disk). Those more specific reports may provide more details about a more specific area, while the more general report provides details about multiple systems.

Following are some of the tools that are known to cover multiple areas.

*stat (Starstat)
Unix type of systems may have systat. Details may be found on CPU usage and syvmstat. Other progarms with some similar names may be fstat and vmstat.
Solutions specific to Microsoft Windows
Resource Monitor
Vista and higher have an informative Resource Monitor that can be reached by running ResMon, from Task Manager's Performance tab, and some other ways. See: Microsoft Windows components: Resource Monitor.
[#prochckr]: Process Hacker

An Open Source (GPLv3) program. See: Process Hacker (Overview page).

[#procexpl]: Process Monitor

Process Explorer (by Sysinternals) was made by Sysinternals (before Microsoft bought out Sysinternals and renamed the suite to Windows Sysinternals). Process Explorer is freely downloadable by Microsoft. See also: section commenting on Process Hacker for an alternative that provides less licensing restrictions. Perhaps see also: Nirsoft Last Activity View or SysInternals SysMon.

SysInternals SysMon

This program will log information about system activity. This can slow down a system (perhaps quite significantly), but also provide lots of information about what a system is doing. Details can include processes (programs) that start, and both TCP and UDP network connections. See: Sysinternals SysMon, IntelliAdmin article about Sysinternals SysMon.

Performance Logs and Alerts

Perhaps see: Microsoft Windows components: ???