AQI - Get air quality updates in your menu bar.
MSI Afterburner - Tool to manage video cards. Shows video card stats (temp, GPU usage, etc.).
CheckMyMac - Hardware and System usage monitor for MAC OS X Intel 64 10.8 +
ASUS GPU Tweak - ASUS GPU Tweak fully implements TechPowerUp’s GPU-Z analysis and monitoring features.
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.
Guru3D - Guru of 3D: PC Hardware Reviews and tests