In the “old days”, if you wanted to display video on a web page you have to upload it to a server
and hope that people had the right media plugin to handle your video format. YouTube changed that by not only hosting and encoding the files, but also by allowing you to embed their viewer into your website. By “playing ball” with the rest of the web YouTube became an integral part of it (cats may have played a role as well).

Something similar happened with maps. By allowing third-party websites to embed their maps (and by providing a formal API) Google Maps evolved from a novel web-version of a classical desktop-app to a structural part of the web.

Currently I’m intrigued by the future of OnShape, a web & cloud base 3D CAD system.
They are openly developing the product at a furious pace (https://www.onshape.com/cad-blog).
At this rate they could soon reach feature parity with their nearest competitors.

But what’s really interesting is that the system doesn’t doesn’t depend on a plugin. It’s all in the browser, pushing the envelope. It is entirely possible for OnShape to allow other sites to use their tech as an integrated service. And they seem to be open to the idea: https://forum.onshape.com/discussion/comment/6562#Comment_6562

Recently I ran into a relevant scenario.  “Enabling the future” is a global network of volunteers that donate 3D print time to create prosthetics for amputees. They have created a great web app, the “Hand-o-matic“. The app let you customise the design of a prosthetic hand and will mail you the generated STL documents based on the supplied parameters.
They use OpenSCAD (an open source CAD modeller) in command line mode to generate still-images of the parts (and to generate STL files).

And embedable CAD system could power a much richer experience for this and many other sites where there is a need for visualising and editing parametric 3D designs.

OnShape has the potential to enable a new kind of web application hybrids (Mashups), that take CAD and the Web to new places.