Just a list of some of my favorite projects and such out there in the world. YMMV, etc...

Replacements for commercial/sucky things

LibreLingo - Open project to create something similar to DuoLingo, but open contribution and federated.

Mastodon - Open source twitter mostly-clone. Pretty excellent stuff.

Nitter - A more privacy dedicated front-end to twitter. Very nice.

Invidious - Not really a replacement, more of a front-end, but still considerably better than youtube itself.

Funkwhale - Decentralized music streaming. Upload your collection and listen anywhere, share, etc... Replacement for Soundcloud.

PixelFed - Very similar to instagram, except open-source and federated (via ActivityPub).

Minetest - Open source game very similar to minecraft but designed for modding. Lots of addable things.