Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.
Prime95 - Prime95 is a program that searches for new Mersenne prime numbers.
SpeedFan - Hardware monitor for Windows that can access digital temperature sensors located on several 2-wire SMBus Serial Bus. Can access voltages and fan speeds and control fan speeds. Includes technical articles and docs.
CPU Frequency Selector - Cinnamon CPU Frequency Selector Applet
xScan - xScan is an application for viewing the behavior of your computer and Mac.
System Stability Tester - System Stability Tester - Test your system's stability and performance by calculating millions of digits of Pi.