SpeedFan is a program that monitors voltages, fan speeds and temperatures in computers with hardware monitor chips. SpeedFan can even access S.M.A.R.T. info and show hard disk temperatures. SpeedFan supports SCSI disks too.Discontinued
The developer promised new versions, but the program seems to be no longer updated. Latest version, 4.52, released in June 2016, can be still downloaded from the official website.
iStat is a series of system monitor apps. It's available in different versions: iStat Menus, iStat for iOS and iStat Server as a companion application to the iStat for iOS app that lets you remotely monitor your computers (free for Windows and Mac).
MenuBar Stats 2 has been reinvented. Completely re-written from the ground up. MenuBar Stats 2 is composed of modules (CPU, Disk, Network,...). Each module can be seen in your menu bar. Each module can be combined in 1 window, or in a separated window.