Hah, interesting! Mine is writing a simple web server (sockets, IO, string handling) and an ultra-simple command line browser (args handling, 3rd party libs for URLs, HTML parsing, etc.)
Mine in similar, I implement an authoritative domain name server. I've done it in something like 20-25 languages now. The only language I've learned in the last decade I didn't write one in is ARM Assembly. I am not that "dedicated" (more like masochistic) enough to try to do one in assembly.
I tend to reimplement the Linux kernel from scratch a few times in every language and go from there onto the X server, desktop environment, networking stack, etc.