RaspberryPi BASIC is an homage to the Commodore 64; a bare-metal operating system that boots directly to a BASIC prompt, similar to early computers. Right now it's incredibly barebones, so expect bugs! The interpreter is based on Adam Dunkels' uBASIC, and the kernel is written in C++.
Download it here.
Open-source project here. To halt a running program, use the Pause/Break key.