CPU Frequency Selector - Cinnamon CPU Frequency Selector Applet
CPU-Z - CPU-Z is a freeware that gathers information on some of the main devices of your system : Processor name and number, codename, process, package, cache levels.
AIDA64 - AIDA64 is a system information, diagnostics and benchmarking solution for Windows PC.
Speccy - Speccy - find the details of your computer's specs. Great for spotting issues or finding compatible upgrades. Download the latest version free.
Udev Discover - A helping tool for udev testers, coders, hackers and consumers.
Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.