We recently attended the OGC/Apache Code Sprint at Camptocamp and improved our experimental support for Collection, Conformance, Tiles and Styles. In the future I would also like to expose the attributes and geometries of the database with the Feature API. We havn't yet tested our implementations with an external client and Pygeoapi would demonstrate interoperability very well.
QGIS might be another client to test against. They’ve implemented OGC API read/write support. Plus, QGIS is great for mocking up a wide range of GIS user scenarios.