HDD Thermometer is provided as freeware, but nevertheless it requires registration. So there are some restrictions in program that will be removed after registration. In any case, you don't have to pay any fee!

Unregistered version of HDD Thermometer will sometimes display nag screen. This is the only restriction and if you don't want to register - you don't have to.

Unregistered versions of HDD Thermometer may be freely copied, transferred to other persons and used on your computers.