Cleese is a project to build a functioning operating system written almost entirely in Python. The basic idea is to have a microkernel mostly based on the Python VM and all other operating system functionality written in Python.

James Tauber and Dave Long successfully built a proof-of-concept back in 2003.

You can see that work, along with some historical information at:

http://code.google.com/p/cleese/

This is a reboot of the project, starting largely from scratch. James Tauber is now joined by Brian Rosner.

To get started, see BUILDING_TOOLCHAIN and SETTING_UP_DISKS.