Tinkercad is a free, online 3D design and 3D printing app for everyone.
the site provide (after registration) access to a basic set of tools for design, model, import and export 3D models for various uses; mainly focused in 3D printing, can be used also for produce AR/VR objects, and or import «plain» SVG files to be converted in a 3d shape.

After the login (the registration give you an “AutoDesk ID” useful for all the other AutoDesk products) you get access to your personal model gallery. All model can be set as «private» or »public» separately, so you can choose to share them with the community. A Thingiverse direct export is also available.

The working area is clean and tidy, the standard dimension of the base is 20x20cm that fit many of the 3D printers in commerce, but, of course, you can change it with your preferred dimensions; you can also change grid snap for more granularity. Simply drop a basic shape from the right toolbar and start modelling adding more shapes, changing lenght, depth and so. Adding and subtracting (using the «empty» shape) shapes you can model many complex objects. All STL export give you a «clean» file, without errors, ready to be printed or placed in a AR/VR environement.