Mirador is an open source viewer for IIIF manifests. The source code is at https://github.com/projectmirador/mirador and an example at http://media.getty.edu/viewers/mirador/?manifest=https://www.nga.gov/api/v1/iiif/presentation/manifest.json?cultObj:id=26
Currently the third party viewer is linked from https://www.wikidata.org/wiki/Property:P6108 . IIIF links are being added to quite a few items so at some point it should probably be better to run our own copy to point to.