The TEMPer is a low cost temperature probe. The deice consists of a usb-serial chip (WinChipHead, a 24xx02 serial EEPROM LM75 temperature sensor CH341) and a LED (the LED is connected to the thermostat output of the LM75). While the software that comes with the device is only for Windows, other solutions do exist. The first one I found was from Tollef Fog Heen's blog, then from Levien van Zon, and finally Stefan Boethke's TEMPer wiki page. The last one works nicely on FreeBSD 8.0-stable, using the uchcom(4) driver.