Movim is a decentralized social platform, written in PHP and HTML5 and based on XMPP. Database backend for caching. Seems suited for not-shared hosting because php-fpm is recommended, as well as Websockets and a long-running daemon (which shared hosting providers like Dreamhost don't like and kill automatically). Seems to require experienced webapp admins because knowledge of the application architecture is required.

Requires that the server support XEP-0060 (pubsub) because message transfer is built on top of it.

A free, open source, distributed search engine using peer to peer communication methods for both search and indexing. Fully decentralized. Written in Java. Install it, start it up, and turn it loose. Can also be used to set up stand-alone nodes for use on intranets, darknets (there's a YaCy instance somewhere in the Tor darknet), and probably single hosts (as a personal search engine).

A collective aimed at activists that uses only OSS software and federated or decentralized software. Offers services that you can request accounts on or set up your own server and federate with them.