I've spent 4 years making a free operating system for calculators (in assembly). AMA!
KnightOS is a free, open source operating system for Texas Instruments calculators. I've been working on it for over 4 years now. (begin nerdy stuff) It's over 25,000 lines of assembly code (which is arguably a pretty tough language to write things with), and it's built with its own toolchain, which is mostly written from scratch in C. It's a passable Unix-like enviornment with multitasking, dynamic memory management, etc (end nerdy stuff) In the past year or so, the project has picked up steam and now there are over a dozen talented people helping me out with it. Maybe some people here will want to get involved, too!
My Proof: http://www.knightos.org/reddit.txt