DanielFalck on the #CAM IRC channel turned me on to a cool feature of HeeksCAD.
Support of STEP files, at least as far as import, is very good in HeeksCAD. McMaster-Carr, which has a truly phenomenal catelog, has an option to download 3d files for many of their parts. They support a number of different file formats including STEP.
If they have 3d data you'll find this link on the part page. Follow it to download and then import the file to HeeksCAD.