The largest thing I wrote was a clone of the parts of "curl" that I use, modifying it slightly so that I can just write JSON to describe what I want to post to a website, rather than the URL-encoded string. This included supporting DELETE, GET, POST, and PUT, and allowing downloading the response only, headers only, or both. I want to add a progress bar and a (admittedly totally unnecessary) GUI, then I'll throw it on GitHub and get feedback on my coding style. I'm enjoying seeing how far I can figure things out without outside help right now, though.
The second largest thing I've written, which I did in a much earlier version of Factor, was to clone enough of Fog Creek Copilot's Reflector that I could use that for testing/developing the Mac versions of the product. That was buggy, disgusting code with piles of stack manipulation, though, and is part of why I quit working in Factor at the time. I wish I still had the code; it'd be interesting to know whether my change of opinion is due to more experience with stack languages, or changes in the language. Probably some of both.
I've gotten interested in stack machines again because of 'brick computing' (re-usable components with a tiny VM in them) and 'fabric computing' (networks of such components, either identical pieces or a variety of them).
It's quite amazing to see how little code you need to get a Forth like environment going on a piece of hardware.