Here's a list of some of the basic helpful tools and apps for system overclocking, stability testing, hardware monitoring, and benchmarking. (All Freeware)
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ORTHOS
http://sp2004.fre3.com/beta/beta2.htm
ORTHOS is basically just Prime95 optimized for dual core chips, and in a user friendly GUI.
SuperPI
http://www.xtremesystems.com/pi/
SuperPI is great for basic memory stability checks, and also for benchmarking overclocks.
Memtest
http://www.memtest.org/
Mestest is a pure memory stability testing tool, that comes in two flavors, the very useful boot-time version that can be put on a bootable floppy disc or CD, and a handy, yet less reliable in-windows version.
Everest Home (Free Edition)
http://www.majorgeeks.com/download4181.html
This is the free version of Everest, which does everything from temp monitoring, to hardware listing, to clock monitoring, and memory benchmarking.
CPUz
http://www.cpuid.com/cpuz.php
CPUz is a very simple and extremely useful system info tool. It displays CPU info and speed, FSB, Memory speed and timings.
GPUz
http://www.techpowerup.com/downloads/822/mirrors.php
Same sort of info tool as CPUz, but for graphics cards.
TAT (for Intel only)
http://www.techpowerup.com/downloads/392/Intel_Thermal_Analysis_Tool.html
TAT is Intel's own thermal analysis tool, monitoring temps, throttle speeds, and even a CPU loading function.
Speedfan
http://www.almico.com/sfdownload.php
Speedfan is another temp monitoring tool, that also includes a level of fan speed control on some motherboards.
A64 Tweaker
http://files.extremeoverclocking.com/file.php?f=106
A64 Tweaker is the same thing as Memset, but for DDR1 A64 based systems
SiSoft Sandra
http://www.sisoftware.co.uk/index.html?dir=dload&location=sware_dl_3264&langx=en&a
Sandra is a nice Memory and basic system benchmarking utility.
Aquamark3D
http://downloads.guru3d.com/download.php?det=673
Aquamark is a system and GPU benchmark program.
Rivatuner
http://downloads.guru3d.com/download.php?det=163
Rivatuner is a GPU overclocking, monitoring, and fan speed control utility.
ATi-Tool
http://downloads.guru3d.com/download.php?det=725
ATI-Tool is a utility for overclocking both nVidia and ATi cards, and has a very handy artifact scanner.
Coolbits
http://downloads.guru3d.com/download.php?det=201
Coolbits is a registry tweak that unlocks overclocking options in the nVidia control panel.
nTune
http://downloads.guru3d.com/download.php?det=1675
nTune integrates itself into the nVidia control panel, allowing GPU overclocking, fan-speed control, and system overclocking options for nVidia chipset motherboards. It also has a nice system temp monitoring feature.
3dMark06
http://downloads.guru3d.com/download.php?det=1297
3dMark06 is a Graphics benchmarking program.
UBCD
http://www.ultimatebootcd.com/download.html
The Ultimate Boot CD - It's like the ultimate boot-time tool kit. It includes a number of benchmarking, and diagnostic tools for hard drives, memory, CPU, etc.