Onshape is the first and only full-cloud 3D CAD system that lets everyone on a design team work together using any web browser, phone, or tablet. Using full-cloud CAD, engineers, designers and manufacturers get secure and simultaneous access to a single master version of their designs without the hassles of software licenses or copying files.
Onshape is a free, professional, and fully-functional CAD software that runs in your browser, meaning your designs are always saved in the cloud, and you can use it anywhere without an installation.
However, for the free user, there are a few restrictions. Free accounts cannot keep their designs private, and signing up for a free account requires a phone number and an organization name. These privacy issues may keep people from using it as their primary modeling software unless they are willing to pay $100/month to upgrade to the Professional level.
Even with these restrictions, it is still a good tool because the developers of Onshape have created a simple, seamless, integrated learning experience for serious 3D modeling.
Some people call Tinkercad more of a "toy" than a "tool", but if you haven't had 3D modeling experience, Fusion 360 can be intimidating for first-time users. Onshape lands in that "Just Right!" range, simple enough for casual users, but with all the features you need to become a professional if you choose to. Their integrated tutorials mean that you don't have to hunt for demos on YouTube or browse complicated manuals.
Even if you don't like your documents being public, it may be worth giving Onshape a try just for the learning process.