What's something that's free that everyone should know about?
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Legal unrestricted, or DRM free, digital media sources and rationale
Almost all software can be a video game if you find something interesting or fun about it
Email is still great for DMs if you only use it for talking to individuals, and not to sign up to things
Some (Slightly Biased) Thoughts On The State Of Decentralized Social Media - TechDirt
In terms of fully free, obligatory mention:
Your library may offer more than books alone, depending on how well supported they are. Borrow music, movies, sometimes even video games. For music and movies they may also offer these to borrow digitally as well via online services they coordinate with.