Until recently, computer-aided design (CAD) software was really only used by engineering companies who could afford to pay thousands of dollars a year per license. The available software, while very powerful, had a very high learning curve and took a lot of training and experience to master. But, with the rise of hobbyist 3D printing, a number of much more simple CAD programs became available.
While these programs certainly helped makers get into 3D modeling, most had serious limitations. Only a few have been truly open-source, and even fewer have been both open-source and parametric. Parametric CAD allows you to create 3D models based on a series of parameters, such as defining a cube by its origin and dimensions. This is in contrast to sculpting style 3D modeling software, which is controlled much more visually. The benefit of parametric modeling is that parameters can be changed later, and the model can be updated on the fly. Features can also be defined mathematically, so that they change in relation to each other.
If you’re looking for a parametric open-source CAD program that can run in your browser, this is it. It’s far enough along that you can use it for real-world (albeit simple) modeling. CAD does, however, still require a certain type of spatial thinking and reasoning. So, if you’re new to the 3D modeling world, it might be worth tinkering with a more learning-oriented tool like BlocksCAD.
[thanks to mmiscool for the tip!]