AlternativeTo now lists over 100,000 apps.
Although AlternativeTo lists both FLOSS and proprietary software it has filters for open source and software running on Linux making it a comprehensive library to search for FLOSS.
@ericbuijs Probably one of my most frequented sites. Great resource.
DuckDuckGo even has a shortcut for it: !alt
Fosstodon is an English speaking Mastodon instance that is open to anyone who is interested in technology; particularly free & open source software.