Many institutions expose all or a large part of their collections through the IIIF Image and Presentation APIs. For getting acquainted with IIIF, it can be useful to test new software or compare usage conventions with these open sources of content. Below is a small selection of institutions that make their content easy to find with IIIF.
All the open content on the Internet Archive is available for experimentation with IIIF. An in-depth post describing these conventions will help you load any IA resource into Mirador or Openseadragon. For example, here is a manifest for Plato's complete works: http://iiif.archivelab.org/iiif/platowithenglish04platuoft/manifest.json Along with its IIIF-based display page. Similarly, an info.json for Van Gogh's Irises can be found here: https://iiif.archivelab.org/iiif/mma_irises_436528/info.json, along with its display page in OpenSeadragon.
The Digital Bodleian provides easy IIIF access to their resources in their search interface. Any result will feature the IIIF Drag and Drop icon.
Stanford University Libraries Searchworks
Stanford University Libraries also make all of their public digital image collections available through IIIF, and exposes their content in search results and PURL pages through Drag-and-Drop.
Harvard Libraries recently exposed all of their image-based results and page-turned books in IIIF. They also host a demonstration site with many resources.
Yale Centre for British Art
The YCBA makes all of their public gallery search results useable through IIIF Drag-and-Drop, and hosts a public endpoint for their content.