Seconded. One of my plethora of hobby projects has been an interpreter or compiler (it changes from month to month) for Common Lisp, for an esoteric platform.

Seconded. One of my plethora of hobby projects has been an interpreter or compiler (it changes from month to month) for Common Lisp, for an esoteric platform. It's a huge amount of fun, and I have learnt so much about software from it.