Open-source alternatives

The following are open-source alternatives (or replacements) to common proprietary applications and services.

Websites
Free alternative	Proprietary counterpart	URL	Notes
Invidious	YouTube	invidious.io	Frontend. Instances often suffer from blockages by YouTube
Mastodon	Twitter (etc.)	joinmastodon.org
Matrix	Discord	matrix.org
Liberapay	Patreon	liberapay.com

Applications
Free alternative	Proprietary counterpart	URL	Notes
Firefox^[a] LibreWolf GNU IceCat ungoogled-chromium	Chrome, Opera, Edge, Safari, etc.	librewolf.net www.gnu.org github.com
Matrix^[b]	Discord, other instant messaging services.	matrix.org

References

↑ Firefox is only a viable, private alternative with post-installation hardening. Please view this guide for more information.
↑ Requires a particular client to use this protocol. Element is one of the most popular clients.