I will start. My new Assimp-Viewer:

Let's shoot! Here is a realtime visualization of the Poppy Ergo Junior robot, in the HARFANG 3D engine. The 3D model is based on the STL files, imported using ASSIMP :

The STL files were converted in our own HARFANG format, using ASSIMP, and the whole simulation is written/animated in Python (code by https://github.com/scorpheus).

Privately I am using assimp for ezEngine (GitHub). Both for static mesh import and animation import. It is much nicer to work with assimp than using the FBX SDK (which I tried at some point). Overall assimp is really awesome, my main gripe has been that new versions often fixed some issues, but regressed in other cases (pretty much always related to animation data from FBX files).

Also, at work we happily use assimp to import gigantic models:
Azure Remote Rendering

Loading a self-modeled Helmet FBX into my custom OpenGL renderer. Assimp made this a lot easier to do!

The current status of the new Asset-Importer-Viewer is based on my renderer. I know that my renderer sucks.

Reading a FBX file with F3D

Credit: theStoff