It’s fine if you have a lot of your PC’s or server’s sensors monitored by Nagios or Icinga. But all these sensors are not very useful if the air conditioner of the room suddenly stops working.
Simple temperature sensors for rooms are not easy to find, if you want them to be connected to a PC or network, and most of them are quite expensive.
TEMPer is very useful in this case. It is a USB stick which can measure the room temperature.
The included software works, but doesn’t have several functions like mail notification, APIs etc. If you need these, use the tool ThermoHID. E.g. if the temperature gets over a certain value, it sends you an email. Unfortunately, it doesn’t run as a Windows service, but you can run it to configure your settings, and then configure a Windows Startup Task to run it automatically.
To control the values, you can also access web pages of the tool via your browser.