http://dmt.bodleian.ox.ac.uk/manifest-editor/
A project we are using IIIF in is https://sinai.library.ucla.edu/ (note a white-listed Google account is required to use, I'll see if I can get a login I can share here).
4Science will share a link describing the road-map for their IIIF viewer. There may be an opportunity for institutions to try out the application and provide feedback.
The link to the roadmap for the IIIF viewer is: http://www.4science.it/en/iiif-image-viewer/ (scroll down for the new features section). 4Science would love to work together with interested institutions to improve the viewer and add useful features to be released to the community.