OpenSCAD, FreeCAD or Onshape.
OpenSCAD is particularly useful when what you want to do is naturally expressed using constructive solid geometry, and even more so if you want to share your work.
But OpenSCAD models only geometry, not operations. If you are thinking in terms of parts and operations, FreeCAD is what you want to use of the free alternatives.
FreeCAD has much richer features and traditional design, but I found it quirky and crash-prone, and sometimes the UI would get stuck and I have had to type python into the console to complete operations.
I have also tried Onshape in the browser, but found it unusably slow, and the dimensioning tool was fairly crude compared to what I was used to in FreeCAD.