For much higher temperatures a infrared sensor that was optically pointed at a source of high-temp equipment that could not be attached to by a standard $150C max temp thermistor device, another plugin would involve a wireless infra-red unit such as:

http://www.yoctopuce.com/EN/products/usb-environmental-sensors/yocto-temperature-ir

https://nz.rs-online.com/web/p/infrared-temperature-sensors/9058783/