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.
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.
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AWS & Azure don't offer free VMs but they offer other always free cloud services. I use Oracle Cloud's Always Free VMs which are great. You can get two AMD-based compute VMs and one Arm-based compute VM with 24GB of RAM. I've been using them for nearly 2 years now. GCP also has a more basic always free VM offering.
Don't forget about cloudflare. With their free caching and cdn a well optimized site can run for barely anything. With GitHub actions and pages you can have a fully static site hosted completely for free.
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.
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...
Edit: It's an app for tracking your baby activities, like feeding, sleeping, diaper changes etc. Fully end-to-end encrypted and fully open source and self-hostable, installable on both iOS and Android.
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.