> not offering (...) GPIO/I2C/SPI/Serial
Nearly every "old PC" does offer built-in easily accessible I2C - as "DDC channel" of a monitor plug - VGA, DVI, HDMI, DisplayPort - all have it. Linux exposes those as regular /dev/i2c-*, so any I2C software that works on a RPi or other Linux SBC, will run with those too:
https://web.archive.org/web/20120903090802/http://www.painty...