CNC machinists tend to congregate on old-school forums, if they're online at all. Those sites can be a good place to ask questions and talk shop. First one that comes to mind is Practical Machinist, but I'm sure there's more out there.
I haven't really seen a lot of sharing sites the way we have for 3D printing, likely in large part because the barrier to entry with CNC machining is much higher in terms of both cost and skill (generally speaking), so there are just way fewer people doing it and they're mostly doing it in commercial environments where they can't share. Models from printing sites may be usable if the creators provide the original design files or steps. STLs typically create artefacts on curved and organic surfaces. in the CAM process
That said, there are some sites on the 2D contour-cutting side of things (e.g. laser cutting, routers). The Maslow Community Garden is centered around the open-source router project of the same name and has some interesting projects on it. I think some of the laser manufacturers oriented towards the hobbyist side of things have similar sites.