PSA: Remember to also check hidden directories you don't even know about for waste of space

Maybe chexkout kdirstat (gui) and ncdu (tui)

the hidden "trashbin", .Trash-$(uid), invented by Ubuntu

This isn't some “idiotic principle invented by Ubuntu”, it just follows the freedesktop.org Trash specification. For many users, it can be really beneficial, see also the spec's introduction:

An ability to recover accidentally deleted files has become the de facto standard for today's desktop user experience.

Users do not expect that anything they delete is permanently gone. Instead, they are used to a “Trash can” metaphor. A deleted document ends up in a “Trash can”, and stays there at least for some time — until the can is manually or automatically cleaned.

If an application like Prism Launcher should use the trash can or delete the files directly is an entirely different question.

I use ncdu. It shows what folders/files are actually eating up all your space.

Doesn't people use a disk usage scanner every once in a while to find out if there's something hogging space?

I'm more of a dust man, myself. It runs recursively so it's easy to pinpoint the culprit.

[Image source: the project's README]

Your desktop environment should have settings for when to clear the trash. You can also use autotrash to clear it automatically.

I use command trash-empty to empty all trashcan of all the users, after each system update. It's a non standard program, but most likely available in your distributions repository: https://github.com/andreafrancia/trash-cli And my alias/function will show each file that is about being deleted (just put it in .bashrc, if you have installed trash-cli, which includes trash-empty):

empty() {
    echo "Files to delete:"
    trash-empty -f --dry-run |
        awk '{print $3}' |
        grep -vF '/info/'
    echo
    trash-empty
}

Edit: After I posted I just realized there is a more straightforward way of doing it:

New and more simple alias:

alias empty='trash-empty -f --dry-run ; trash-empty'

This searches all trash cans, lists all files it has found to be deleted, then lists all directories it looked under and then asks if you want really delete. With trash-empty -f it deletes without asking.

@30p87 @hyperreal ncdu is great for that

All .Trash-1000 files appears in your trashbin no? At least is how is working for me.

Isn't it common that file managers have a trash bin? Thunar does.

ncdu

I did this yesterday while trying to make a backup before formatting and then updating my work rig and I'm SO GLAD that the decades have taught me to take knee-jerk/automatic actions that forces me to make entire volume backups; otherwise this last month of work life would have been AWFUL!!!

This is why I use gdu on servers

Hard delete used to be in the drop-down along with move to Trash, since I rarely accidentally delete things and when I do I have backups, I much preferred that arrangement.

On servers I primarily use a terminal, only for things like virt-manager do I ever fire up a desktop so on servers mostly moot.