Modelling 3D graphics is as much about the tools, as about the talent of the designer. In one end of the spectrum, the likes of SolidWorks are found. In another corner, the more programatic approach of OpenSCAD is found attractive. Some others might want a cloud based solution. Then TinkerCAD and 3DTin provides something for your needs.
The TinkerCAD tool gives a nice on-line sketching tool that can produce STL files suitable for printing. Having created an account and logged in, the interface does required WebGL, but that is supported by most modern browsers and platforms
Models are build by adding or subracting basic geometrical shapes from the existing model. The system of having a working plane makes it easy to add shapes at an angle. You are also given access to basic tools for measuring and panning around the scene.
The people over at 3DTin offers a nice little tool for creating basic 3D models. It can run either in a limited mode, requiring the HTML5 canvas component, or in a rich mode based on WebGL.
In the basic mode, you are limited to drawing coloured boxes, which the rich mode offers more advanced primitives. It is also possible to import 2D images and extrude them into 3D shapes.
Models can be saved and loaded to 3DTin’s servers if you create an account and log in. It is also possible to browse the designs of others, as well as provide your own models to your peers. When you are happy with a model, you can export it as STL, or other formats. This does, however, require you to save the sketch, so you need to register and account.
As you can tell, on-line CAD tools rely on the availability of WebGL. The support for WebGL is growing. For instance, Google Chrome, supports WebGL across Windows, OS X and Linux. However, requirements are also placed on the graphics drivers. The same applies for Firefox.