Friendica
Oldest active Fediverse software — started 2010 (before ActivityPub existed). Facebook-like UI: news feed, friends, events, photos. PHP. Supports multiple federation protocols: ActivityPub, Diaspora, OStatus, RSS.
Bridge function: can crosspost to Twitter, RSS feeds, even Facebook (when the FB API allowed). One of few fediverse software with calendar + events. Less popular than Mastodon (UI feels like 2012-Facebook), but stubbornly active community. Forerunner to the entire ActivityPub movement. Modern use: existing Friendica instances convert to Mastodon, but the core has a dedicated community. RedMatrix → Hubzilla is the sister project.