BleachBit

Open source app Bleachbit is works wonders for removing all kinds of residual files and other traces of activity on your Windows or Linux computer. It is therefore a privacy- and security-enhancing tool. Security researcher Bruce Schneier, who handled some of the leaked Snowden files, has used Bleachbit (amongst other things) to keep his computer clean and secure against NSA spying.

Bleachbit is also allegedly the app Hillary Clinton (or her staff) used to erase those infamous emails. Hillariously (pun partially intended), during one of the 2016 US presidential debates, Donald Trump accused Hillary Clinton of "bleaching" her emails, calling it "a very expensive process".

Rest assured that Trump made that up: Bleachbit is open source, free, and costs nothing to use. Oh, and also you don't have to pay for it.
(Except perhaps that its use might attract misleading statements from Donald Trump.)

Bleachbit is able to go very deep and will delete a lot of things you wouldn't even know your computer keeps. I have never encountered any situation in which data has been deleted I didn't want it to. It does not break your registry or similar. It has also solved problems when not being able so install certain software.
A second feature besides just cleaning your system is that you are able to "shread with bleachbit" which will delete files and its digital footprint on your hard drive by changing its name and then deleting again. I recommend it to all my customers and friends.

Best alternative to CCleaner

Open source and with no adware like some other cleaning software. It does great job cleaning fast and efficient!

Use this on GNU/Linux OSs it cleans up the dust i leave behind.

OMG, BleachBit just deleted 196 gigabytes of Windows temporary files. I was already thinking about buying new hdd for my old PC, but BleachBit saved my money and time.

helped me clean more 15g of sessions and temporary files
works great

Don't suffer from any adware added ! so now better than cCleaner !
I consider it still in beta since few important feature are missing (cannot change the language for exemple)

Its great and it kinda cleans better than bigger optimization apps

Eating my CPU but no window is present. Kinda like a malware, except this pretends it helps you.

Only shreds one file at a time via the right-click context menu. Other file shredders like Eraser and CyberShred do all the files you've selected. You can do multiple files at once with Bleachbit, but you need to open the program first and click-drag the files into it. Very jarring and clunky once you've had the convenience of "right-click > Shred" with other file shredders. Not sure how it is with cleaning compared to CCleaner, I only wanted it for file shredding.

[Edited by GregariousJB, July 19]
EDIT: Forgot, I also used it to wipe free disk space, which seemed to work well.

Sorry, Bleachbit. I'd love to hail you above CCleaner now that it's ever more an advertising platform disguised as an application but your Open Source roots are glaringly visible.

You look primitive bordering on primordial. The dark mode is nice but the Win10 "theme" is just pure black with white tick boxes. That's Windows '95 not 10.

You have a go at explanation for certain things but in a way that still expects the user to understand what you're referring to (which I've found is a VERY Linux approach) e.g: an option for Thunderbird, Index, is described by three words: "Delete the files". Which files? Attachments? Logs? The application files???

You're slow and your application list is woefully out of date. Who in this day and age uses Winrar or Yahoo Messenger??

Ccleaner simply offers more, a one-stop shop. I don't deny you'll find a niche and are probably good at what you do once people can work out precisely what that is but you're no replacement for Ccleaner. That's like offering up Abiword as a replacement for the full MS Office suite.

[EDITED 22/07/2020 = tidy-up]

I have been using Bleachbit for years on Linux and I only dared to use this software on Windows 10 about two years ago... my disc C: was two-thirds full although I regularly use Ccleaner. I was wondering if Bleachbit could free up more space harmlessly...
This was successful ! now my C drive's a little over a third full.

The only small problem is that you will have to redefine some settings in Windows 10 : the default programs and the arrangement of the desktop icons.
For the desktop icons, I use a little soft named Desktop Ok (you just need to save the desktop configuration from this software before launching Bleachbit so that you can restore it after Bleachit has grouped all your icons on the left side of the screen).
To avoid this tow actions (returning to Windows default software and moving desktop icons), I suppose there would be some checkboxes to be unchecked in Bleachbit but I could not find which ones...

[Edited by adresweb2b, July 05]

It's privacy focused, does not promote bloatware, and fairly straight forward to use but is also very comprehensive. A great replacement for tools like Ccleaner which have lost their way.