It's like an old-school mercury thermometer, but looks way cooler (or hotter)!

The Metre LED Thermometer uses an Atmel microcontroller and an on-board temperature sensor, along with an array of colorful LEDs, to indicate the current ambient temperature.  Each LED represents a temperature change of 4 degrees F, and the PCB is marked along the top from 60 F to 104 F.

The schematic and board were generated using Eagle and the code was written in the Arduino environment, and it's all open source.  It's fun and easy to build.  It stands alone as its own project, but is also a great base for any other temperature-sensing project.  Additionally, all the I/O on the microcontroller (except for one ADC pin) has been brought out to external pads, so the board can also be used as a general purpose AVR development board.