Lukas F. Hartmann decided that there simply weren’t enough operating systems available for the Raspberry Pi and endeavoured to write a brand new one from scratch. Using his past experience, he has managed to create an alpha build of InterimOS using the LISP language. You can read a lot of the technical background on his website, where you can also download the image to install on your Pi. Bear in mind this is an alpha release so your mileage may vary!