A self-hosted website to manage all your collections, of pretty much any kind. You can choose to share your collections with other people, while retaining full control on what they will be able to see. Other people can be invited to manage their collections independently on your server.
Backed by Postgres or MySQL. Written using the Symfony framework.
Don't know if there's an API or not.
Digikey's choose-your-own-adventure subsite for identifying connectors.
CKAN is open source data management portal software for extremely large data sets, on the scale of entire governments. Used to publish, curate, share, search, and find datasets of just about any kind. Does visualization and metrics. Source code on Github (https://github.com/ckan/ckan). Written in Python, uses Postgres as its back end. Can interface with Apache Solr for search and indexing.
An online catalogue of Victorian, Edwardian, vintage, and steampunk accoutrements, such as sextants, swords, compasses, and other nifty things.
A clone of Librarything. Used to inventory your personal library. Integrates Wikidata to get more information for the contents of your library.
API: https://api.inventaire.io/
Github: https://github.com/inventaire