I made something for Mastodon: A gallery mini-app.
https://source.joinmastodon.org/Gargron/gallery
You can simply drop it in your Mastodon server's public folder, and it will show images from your server in a mosaic grid with infinite scroll. Supports some configuration as well, e.g. whether to limit it to only local images, or a specific hashtag, or which server to fetch from.
@neontiger mastodon has an open api, so anyone can write apps or plugins for it, like Gargron's gallery tool or https://pinafore.social/
Mastodon federates using the ActivityPub protocol, as does Pleroma, Peertube, plume, and pixelfed. Anyone with the appropriate skills can build their own social network that communicates with the rest of the fediverse using ActivityPub: https://www.w3.org/TR/activitypub/
You can also contribute code to mastodon itself through github: https://github.com/tootsuite/mastodon
Eugen Rochko
Leonardo
Satsuma [moved]