A free, if rather simple, operating system for the TI-83 Plus family of graphing calculators...
See the SourceForge project page for what information is currently available, and to download the latest files.
At the moment, the only supported assembler is Thomas Anderson's TASM; at some point I hope to work out a more convenient option, but for now this is it. For the moment, I'll make the following offer: if TASM is not available for your platform, and you have a modification you'd like to make, e-mail me the source and I'll assemble it for you.
Contact Me (FloppusMaximus)