Most PC motherboards have a built-in I2C bus that connects to sensors for CPU temperature, ambient temperature, fan RPM, and other interesting values. Some motherboards have connections that could easily be hacked by the robotics experimenter (e.g., replace the chassis intrusion switch with a whisker/bumper switch; or replace a fan RPM sensor with a wheel position encoder). A software package called lm_sensors is available for Linux that provides an easy way to read the sensor data from the I2C bus. It's free software released under the GNU GPL.