Depends on your use case; Tinkercad runs in browser, and while is very 'simple', is a great starting point for people.
Meshmixer isn't specifically for printing, but it's free, and provides a nice suite of freeform poly modeling tools (I used it on my mac in the past).
If you're looking to do more 'real cad' work, I used Fusion360 on my mac back in the day. Free for hobbyists, but extremely powerful.
OnShape also runs in your browser, but I'm not sure what the cost is.
Finally, Blender is free, runs on mac last I checked. Mainly for poly modeling, not cad, steep learning curve, but can do a lot depending on your needs.
Makerbot Replicator 2x / Sailfish on Mightyboard
Custom C-Bot / RepRap Firmware on RADDS
Custom Tevo Little Monster / RepRap Firmware on Duet