Free S.M.A.R.T Hard Drive Monitor for Windows 98, XP, Vista, 7, 8

S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology; often written as SMART) is a monitoring system for computer hard disk drives to detect and report on various indicators of reliability, in the hope of anticipating failures.

Download a free for personal use copy of PassMark DiskCheckup ™ utility here. Great commercial grade and supported software. 

For open source and alternatives see Wikipedia's list https://en.wikipedia.org/wiki/Comparison_of_S.M.A.R.T._tools

In order of preference; 

1. CrystalDiskInfo - build by Japanese developer Noriyuki Miyazaki a Microsoft MVP.
   
   Great utility, that show temperature per HD in system tray. It supports HDD/SSD utility software which supports a part of USB connection and Intel RAID. Adware removed now,

2. GSmartControl - Hard disk drive and SSD health inspection tool * 

   GSmartControl is a graphical user interface for smartctl (from smartmontools package), which is a tool for querying and controlling SMART (Self-Monitoring, Analysis, and Reporting Technology) data on modern hard disk and solid-state drives. It allows you to inspect the drive's SMART data to determine its health, as well as run various tests on it.
3. HDD Guardian provides a Windows front-end for smartctl, a utility which monitors your hard drive(s) and SSD(s) for health status, taking advantage of S.M.AR.T. tool as provided by https://www.smartmontools.org/. But no individual temperature gauges per hard drive.