Recommended No-Cost Software

[LongRunner]

Here is software that has proven useful to us, and is legally obtainable without spending any money. (Why didn't I just title this thread "Recommended Free Software"? Because the word "free" has two distinct meanings which are otherwise, inevitably, blurred together; availability at no cost, as in "free beer", and liberty, as in "free speech". All of what is called "free software" in "common" parlance, is free in the "free beer" sense, but only some of it is free in the "free speech" sense. Richard Stallman explains in more detail and depth.)

Where available, links are to the software author's own site, as this is (normally) the safest against malware (and general crapware). If the official location is no longer available/functional, links to Wayback Machine or a reputable software archive will be used instead. HTTPS (secure) addresses are used, where supported by the relevant site(s).

Categories (click a link to go to a post)
Internet (IM, Torrent, Web)
Multimedia editing
Multimedia viewing/playback
Office
Miscellaneous (anything not in another category)

If you have additional suggestions (I'll be particularly interested in free-as-in-freedom, or at least open-source, alternatives to closed-source programs listed), questions, malware warnings, or just general commentary, please reply with them/it. However, do not waste screen space conversing about things irrelevant to this topic.

History of this thread:
v0.1 (initial) listed foobar2000, GIMP, InfraRecorder, Inkscape, LibreOffice, Malwarebytes, Opera 12, Paint.NET, Pidgin, qBittorrent, and SRWare Iron
v0.2 (2017-06-23) added 7-Zip, Audacity, Notepad++ and SumatraPDF
v0.3 (2017-08-22) added SpeedCrunch and XYplorer
v0.4 (2017-10-17) deleted SRWare Iron (as it seems to be a bit of a scam)
v0.5 (2018-02-11) made separate posts per category, this post becoming the index; added Pale Moon
v0.6 (2018-02-13) added uBlock Origin
v0.7 (2018-02-14) added WinCDEmu, mentioned (but not explicitly recommended) Easy 7-Zip
v0.8 (2018-03-09) replaced Paint.NET with Pinta (I could have done this earlier, but stuck with Paint.NET out of habit )
v0.9 (2018-03-13) added (good old) DVD Shrink
v0.10 (2023-11-26) added Return YouTube Dislike, Audacious, Aegisub, Gnumeric; fixed a few broken links
v0.11 (2023-12-18) added license information after the software names
v0.12 (2023-12-19) edited post links so they don't have to reload

[LongRunner]

Instant messaging client
Pidgin (GNU GPL 3.0; Windows, Mac OS, GNU/Linux)
Warning: The downloads are hosted at SourceForge, which has gotten a bad reputation lately due to malware/crapware issues, so I'd recommend running a malware scan on anything downloaded from them – if you are willing to keep downloading from them at all…

Torrent client
qBittorrent (GNU GPL 2.0/3.0; multi-platform) – an open-source, ad-free alternative to μTorrent. I urge anyone still using μTorrent to switch over as soon as possible.
(Warning: "Screenshots" link goes to SourceForge)

Web browsers
Opera 12.18 (proprietary; Windows) (third-party site, but links to Opera's own FTP server) – officially a "legacy" version (made for the sake of those still on Windows XP SP1), but regarded by many as the last true Opera (the new "Opera" being just another Chromium derivative). Following its release, Behemot featured it in our News section. A pity it's only for Windows, as I'd wager quite a few people would have wanted a GNU/Linux version; GNU/Linux (and Mac) people will have to either make do with 12.16, or use something else altogether.
Pale Moon (Mozilla Public License 2.0; Windows, GNU/Linux) – forked from Mozilla Firefox many years ago (still compatible with many, though by no means all, Firefox add-ons), this looks like a well-optimized modern browser (I'm using it now, and already it seems to be one of the better ones I've tried).

Browser add-ons
uBlock Origin (GNU GPL 3.0; Chromium, Firefox/SeaMonkey/Pale Moon; forks available for Edge, Safari) – efficient "wide spectrum" blocker
Return YouTube Dislike (GNU GPL 3.0; Chromium, Firefox, Edge) – if you hate the likes-only nonsense on mainstream social-media as much as I (and especially HumbleRando) do, at least you can reveal the dislikes on YouTube videos (though not the comments).

[LongRunner]

Multimedia players
Audacious (open source; GNU/Linux, Windows) – not as sophisticated as foobar2000, but FOSS and works well enough for general use.
I still use v3.10.1 as v4.x moved from the reliable GTK interface to the seductive-but-buggy Qt (you can choose GTK but it still doesn't work as well as v3.10.1).
foobar2000 (proprietary; Windows, Mac OS X) – audio-only player with a wide range of available add-ons and customisations. Attention, audiophiles – if you want to compensate for frequency response aberrations, I strongly recommend this plug-in equalizer which provides much cleaner sound through-put than the built-in unit, and also allows finer adjustments.
VLC (GNU GPL 2.0; multi-platform) – extremely popular, versatile player for a wide range of formats. Note: You may need to tweak some settings to get the correct black and white levels in certain video formats, including DVD-Video.

Picture viewer
XnView (proprietary, Windows; XnView MP also supports Mac OS X and GNU/Linux)

PDF reader
SumatraPDF (GNU GPL 3.0; Windows) (also supports ePub, MOBI, CHM, XPS, DjVu, CBZ, CBR)

[LongRunner]

Audio editor
Audacity (GNU GPL with documentation under CC-BY 3.0; Windows, Mac OS, GNU/Linux)

Raster image editors
GIMP (GNU GPL 3.0; multi-platform) – a powerful program that mainly seeks to be an alternative to Adobe Photoshop.
Pinta (MIT License; multi-platform) – a substitute for Paint.NET, which even has a few features Paint.NET lacks.

Subtitle editor
Aegisub (open source; Windows, Mac OS X, GNU/Linux) – if you want to make or tweak anime fan-subs (whether mainstream or Mitsudomoe), this is the tool.
Note that Mint 21 (or Ubuntu 22) no longer supports it, so I'm staying on Mint 20.3 for as long as I can (as none of the alternatives are as functional)…

Vector graphics
Inkscape (GNU GPL 2.0; Windows, Mac OS, GNU/Linux)

[LongRunner]

CD/DVD authoring
InfraRecorder (GNU GPL 3.0; Windows) - while a bit neglected update-wise, it is one of the few utilities in its class to remain crapware-free.
(You will have to go elsewhere if you want Blu-ray or HD-DVD support, unfortunately.)

DVD-Video ripper
DVD Shrink (proprietary; Windows) - an old but "classic" program that I used until ditching Windows; it might not crack every DVD, but it seems to work on most.
(There is a new official site, but it doesn't work properly so don't bother going there.)

Compressed archive manager
7-Zip (mostly GNU LGPL; Windows, with limited ports to other platforms available) (Warning: Some links on the download page go to SourceForge)
A modified version (called Easy 7-Zip) is available that's meant to be user-friendlier (I've not tried it), but updates lag behind the official release.

File manager
XYplorer (proprietary; Windows) (updates to the free edition have been discontinued since November 2016, but the last version is still available for download)

Malware detection and removal
Malwarebytes (proprietary; Windows, Mac OS) - widely recommended in this category. The free version of Malwarebytes 3 provides basic anti-malware/spyware and anti-rootkit functionality forever, and a 14-day trial of the "premium" features. As has been customary for Malwarebytes, the free version is (after the trial end) limited to manual scanning on demand. To be fair, classical anti-virus software has proven increasingly helpless against new threats - and not only can it cause inadvertent damage with false-positive triggering, it can even open up security holes of its own. On a computer likely to be used by untrained people, real-time anti-virus software may be justified (although such people might not even be able to operate the anti-virus correctly!); however, if you're an experienced user (who knows how to avoid contracting malware) configuring a PC just for yourself (and perhaps other tech-savvy friends or family members), it is increasingly questionable.

Optical disc emulator
WinCDEmu (GNU LGPL 3.0; also creates ISOs)

Scientific calculator
SpeedCrunch (GNU GPL 2.0; Windows, Mac OS, GNU/Linux)

Text editor
Notepad++ (GNU GPL 2.0; Windows)

[LongRunner]

Office suite (word processing, spreadsheets, presentations, and related)
LibreOffice (Mozilla Public License 2.0; Windows, Mac OS, GNU/Linux) – a free alternative to Microsoft Office and its kin, with some (reverse-engineered) ability to open the former's documents. It still has a few glitches, unfortunately (the most annoying being the text positioning on-screen going out of whack), but I'd take it over reinforcing Microsoft's stranglehold.

Spreadsheet (stand-alone)
Gnumeric (GNU GPL 2.0 or 3.0; GNU/Linux, BSD) – if you want to number-crunch with the utmost efficiency and don't need the nicer text rendering or greater customizability of LibreOffice Calc, go for it. (Some third-parties compile it for Windows, but use at your own risk.)