Here we have a list of software for different purposes that are (mostly) Open Source (and mostly free).
Software for Creative & Design Purposes:
Krita - Free and open-source raster graphics editor ~ equivalent of Photoshop, or more for illustration and animation
Gimp - Free & Open Source Image Editor ~ equivalent of Photoshop
Scribus - Libre Desktop Publishing ~ equivalent of InDesign
Kdenlive - Free and open-source video editing software
DaVinci Resolve - Video Editing Software (not open source, but available for Linux)
Social Media Alternatives:
Pixelfed - Image sharing app ~ equivalent of Instagram
Mastodon - Microblogging ~ equivalent of Twitter. Have a look at the instances and choose what suits you best, rather than choosing the social.mastodon
Video Publishing:
Hyper8 - Static Website generator for video publishing, instead of Vimeo and Youtube
Music Publishing:
Mirlo - Music Sharing Platform instead of Spotify and Bandcamp
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Newsletter Alternatives:
Text Editor
FTP, SFTP, WebDAV, and Cloud Storage Browser
Static Web Site Builder
Self-hosted Cloud Suite
Hosting Options
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Netlify (free, but not open source)
CMS - Content Management System
Digital Publishing Tools/Platforms
Write Freely can be self-hosted and used for blogging, it has a (RSS) feed and can be published to fediverse.
Note Taking
Joplin Can be self-hosted to use it across devices
Cryptpad Online pad and online form (instead of Google Forms)
Book Stack Collaborative note taking tool that can be self-hosted (actually a Wiki software)
Peer-to-peer File Sharing
Soulseek - P2P file sharing, mostly for music
RSS
Fresh RSS - Free, self-hostable feed aggregator
Ad-blocking
Pi-hole - Network-wide ad and tracker blocking
Self-hosted Book Tracker instead of Goodreads
Other tools