Fixed-gear/single-speed calculator for bicycles
This project is the source for the website at https://eehouse.org/fixin.
It's an extremely specialized calculator that helps bicyclists with frames with vertical dropouts figure out what gears (combinations of chainring and cog) will allow them to run their frames as fixies or single-speeds.
Did I mention it's specialized? But for those who need it it's proven useful. It still brings a handful of visitors to my site every day, and has been doing so for 30 years. It's been around for that long in one form or another, always free to use, but this is the first time I've published the source. Enjoy.