What free things online should everyone take advantage of?

If you're in the US, you can sign up for Informed Delivery with USPS so you know when to expect incoming important mail. This can help if mail theft is common in your area.
- UPS and FedEx also allow you to sign up for similar alerts based on your address so you don't have to necessarily rely on tracking numbers for everything.
- It also helps if you are too lazy to bundle up to go outside to check when it’s cold as nuts, but are waiting for something specific. 😁
  And if you just never check your mail (I get almost exclusively junk mail, so I check it every week or two. All my bills are autopay, and all communications are paperless when possible).

Open Library allows you to digitally borrow a ton of books for free. It's not the greatest experience since the books they own are scanned and not digital copies, but it's good enough, and their catalogue is not half bad.
- Why would you need to borrow and return in this scenario?
  
  Just legality. These are paid books that they can't give away for free, but acting as a library they can let you borrow them, read them, and "return" them.
  In practice you'll rarely feel this system because you can just re-borrow it whenever you want to read it.

https://www.khanacademy.org/
Anyone can learn anything. For free. Education is a human right. We are a nonprofit because we believe in a free, world-class education for anyone, anywhere. Instead of ads or subscriptions, we are supported by individual contributions from people like you. Please join us today.
- Are you the original Khan?
  
  The vertical line next to the text means it's a quote

Internet Archive - Very useful for general usage internet but not for specific needs (check fmhy).
Have I Been Pwned? - Periodically use this to check whether your email/online presence is compromised or not.
Bitwarden (or if you want to self-host it use Vaultwarden) - Personally I used this password management so I can keep my pass and you can auto generate passwords with custom parameters if needed.
Curated OSINT resources - You want to ID someone online? this resources can help you to do so, though you also need to understand the basics. It may not work if person you're looking for does not have a much online presence.

MOOCs.
They're free unless you want a "certificate", which has no practical value and is mostly useful if you want to motivate yourself a bit more and want to support their business model.
Go browse Coursera and edX and learn to your heart's content.
Also, if you're coding, watch recordings of conferences about your programming language, e.g. CppCon/C++Now/CppNorth for C++, PyCon/PyData for Python etc...

If you’re not a fan of your current email provider, I’ve created one that automatically organizes your email and helps prevent spam and phishing:
https://port87.com

Awhile ago, a user on r/casualuk posted a list of UK based stuff that was either free or cheap (food/entertainment/etc). They mirrored it in a few places and I bookmarked the github mirror. Doesn't look like it's been updated for a year or so though and I ain't prepared to head to reddit to check the original.
- I just checked the Reddit thread and the OP took it down in protest of Reddit's API changes.
  
  Ah OK. Glad I bookmarked the mirror then.

yt-dlp for downloading video/audio from a number of sites, not just YouTube.
Screw YT Premium.

If women want free penis pictures and rape threats they can visit any 'incel' community and state "I am a woman" then sit back and wait.
I do not recommend this.