If you are looking for a physical display...

http://www.frozencpu.com/cat/l3/g44/c133/s456/list/p1/DIY_Modders_Dream-PC_Tools-Thermometers-Page1.html

I actually use one in my music studio to monitor heat sink temps on my amp rack. Many of them have alarms that you can preset to a set temp limit.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car