Thumbor and imgix seem nice products. I'm also working on another very similar product, http://thumbr.it/. I guess our main differentiator is that we also do thumbnails of documents and we allow the user to create their own effects on the photos. However, we don't have any of the text options available on imgix, or the smart crop that keeps faces in the image.

From a quick glance at Thumbor, it seems it also offers object detection (like faces), which is a nice add-on. Nice post, I didn't knew about this project, even though I'm working on the same space!