This is different than frame geometry, which is what is being discussed in this 99spokes article, but it's quite useful for anyone that likes to play with their fit or wants to transfer their current fit to a new frame.
It's a work in progress, but the app can be found here: http://app.mybike.fit.
A Free online version is also available. It takes >5min to load, just be patient.
rattleCAD <= 3.x was fully free & open-source, but actually rattleCAD 4.0 is proprietary with freeware/demo version & few subscription modes.[1]