3DTin is a free web-based program for creating three dimensional models. 3DTin provides templates that you can use to develop models or you can build a model completely from scratch. The service is completely free to use as long as you allow your models to be labeled with a Creative Commons license and put in the 3DTin gallery. The gallery is a great place is to find examples of what can be created in 3DTin. Models that you create can be exported for use in other modeling software.
Before you rush to try 3DTin you should know that you do have to be using a modern web browser that supports WebGL. For example, 3DTin will work with the latest versions of Chrome and Firefox but will not work with older versions of Firefox or Internet Explorer. 3DTin can be used as a Chrome Web App too.
The video below offers a short introduction to 3DTin, more tutorials are available on YouTube.
Applications for Education
3DTin could be a great web tool for an little engineering or architecture activity in which students develop and test various structure designs.