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.
Bots:
Makerbot Replicator 2x / Sailfish on Mightyboard
Custom C-Bot / RepRap Firmware on RADDS
Custom Tevo Little Monster / RepRap Firmware on Duet
For the past two years, I've been using Freecad for non-consulting engineering work and I recommend you give it a try.
Version 17 development is close to release and I find it much better than the released 16 version. I use the pre-release 17 version on OSX Serria and Freecad is fast. They have a good/helpful forum...
gcodestat integrates with Simplify3D and allow you to
Calculate print time accurately (acceleration, max speed, junction deviation all taken into consideration)
Embed M117 codes into G-Code
Upload your G-Code directly to Octoprint
open source and unlicence
I finally ended up using openscad....though it seems it hasn't been updated for a while...
but much more control than the sluggish and imprecise GUI on ViaCAD (o;
I personally like the way that you model the design in a text editor...just like in the good old days on SunOS and render then in BMRT (o;
Now I just have to find out why the first print on my K8400 is rubbish....have read that the original extruder steps/mm is wrong in Velleman's firmware.