VOCALOID alternative, but don't expect the same voicebank, like Hatsune Miku or Po-uta.
Just in case you don't know what is Vocaloid, it's basically a voice synthetizer software.
There's also another FOSS voice sythetizer (talk only, no singing) called VOICEVOX. This one is Japanese only, but it has popular Zundamon official voice bank! https://github.com/VOICEVOX/voicevox
Powerful subtitling software, used by amateur and professional.
Probably the most used software in fansubbing scene.
Some company like 
Crunchyroll or Muse Asia are actually using this software.
Powerful video editor with KDE principles: "Simple by Default, Powerful when Needed"
Use this instead of pirating 
Wondershare Filmora. I know some of you do. Yes, some my friends did this all the time and ended downloading trojan.
Free, open-source video editing with multi-format timelines, frame accurate seeking, and robust capture support up to 4k for diverse projects.
Free cross-platform DAW supporting VST plugins, MIDI, and lossless audio. Offers music loopers and sequencers with privacy, no ads, and offline capability.
Free open-source multi-track audio editor and recorder with 32-bit float processing. Supports many formats and plugins for Windows, macOS, Linux.
Originally forked from 
MusE Sequencer, now becomes one of the most user-friend notation music software.
Ever tired managing art upload to many various sites? This software (might be) your solution.
Supports various sites, from 
X, 
DeviantArt, 
pixiv, 
Bluesky, and so on.
Fediverse instances such as 
Mastodon, 
Misskey, 
Pixelfed, and 
Pleroma also supported.
There won't be any support for any Meta platform, including Facebook, Instagram, or Threads. (Require paid API?)
Browse, preview, and install fonts from Google Fonts with a sleek interface, privacy-first font previews, code for CDN or NPM use, TTF downloads, and easy filtering.
Turns tablets or smartphones into graphic tablets or touchscreens, controls mouse, mirrors screens, supports stylus with pressure tilt, and multi-touch on Linux.
Display calibration and profiling tool.
The latest update of the official version is in 2019. There's maintained community forks on https://github.com/eoyilmaz/displaycal-py3
Danbooru is an English language imageboard and image hosting website focused primarily on anime style illustrations. It was launched in 2005 by a programmer known as "Albert" and is frequently described as one of the earliest and most influential "booru"...
Mix any color from your reference photo with the paints you own 🎨 Outlines, grids, tonal values, limited palettes, photo editing. Free web app.
Open-source PDF viewer/editor using Qt framework with C++ library, multithreading, hardware rendering, encryption, text layout, and form tools.
ImageMagick is a software suite to create, edit, and compose bitmap images. It can read, convert and write images in a variety of formats (about 100) including GIF, JPEG, JPEG-2000, PNG, PDF, PhotoCD, TIFF, and DPX.
G'MIC is a full-featured open-source framework for image processing, distributed under the CeCILL free software licenses (LGPL-like and/or GPL-compatible). It provides several different user interfaces to convert / manipulate / filter / visualize generic image datasets...
Multi-platform document viewer supporting PDF, EPUB, comics and images with annotation tools, sidebar navigation, signature verification, and text-to-speech.
This section will feature general and specialized engine.
Capable professional 2D and 3D game engine. Even Sonic Colors: Ultimate was developed using Godot.
For beginner, this engine doesn't have visual scripting like Construct or 
GameMaker, but Godot's GDScript programming script is relatively easy to learn. They remove the visual scripting on previous release, as it needs a better implementation.
As in May 2023, Godot regularly becomes the top used game engine on itch.io, surpassing 
Unity.
General 3D engine.
Compared to 
Godot Engine which only focused on game creation, O3DE has wide support, from robotics, virtualization, and other industry needs.
Hosted by Linux Foundation and backed by various company, such as Amazon, Huawei, Oppo, and RedHat.
Wicked Engine is an open-source game engine written in C++. The main focus is to be easy to set up and use, light weight, high performance, and graphically advanced. The full source code is provided with the MIT license, which means, anyone is free to use it for anything without...
Phyton-based visual novel creator. Have relatively easy interface that beginner programmer can easily pick it up.
Ren'Py powers a lot of successful visual novel that are ported on various platform, such as Higurashi series, Clannad and PLanetarian (according to 
The Visual Novel Database: https://vndb.org/v?f=N18fwRen_7Py-&s=n4w )
No-code game engine.
While it is free and open source software, the official software release require you to pay €4.99 or €9.99 per month for more feature.
For complete comparison, visit: https://gdevelop.io/pricing
Only use this if you're a complete beginner (learning Godot Engine is a better long-term investment if you want to scale up your game design.)
Eldiron is a cross-platform creator for classic retro role-playing games (RPGs). Its primary goal is to enable the creation of RPGs reminiscent of the 1980s and 1990s while incorporating modern features such as multiplayer support, procedural content generation, and more.
Open-source library for JS/TS enabling comic content integration, customizable comic-style pages, interactive navigation, and multi-language support.
Coollab is a free, open-source, node-based software for creating generative visuals in real time. It is designed to be very intuitive and easy to use, yet very powerful and flexible!
PraxisLIVE is an open-source, visual programming environment. Experiment with and create projects using a visual patching system coupled with the power of programming in Java, Processing and GLSL.
NodeBox is a new software application for creating generative art using procedural graphics and a new way to approach graphic design.
TiXL is an open-source solution for creating interactive 3d content and animations. It combines the best aspects of animation, compositing, and coding into a lightweight and intuitive UI. Your designs can be shared as executable or rendered to movies.
Processing is an open source programming environment and library built to handle images, animation, and interactivity. It is used by students, artists, designers, researchers, and hobbyists for learning to code, prototyping, and producing full applications.
...but I'm not sure regarding the quality, as the last time I used it was probably more than decade ago or probably never.
novelWriter is a Markdown-like text editor designed for writing novels and larger projects of many smaller plain text documents. It uses its own flavor of Markdown that supports a meta data syntax for comments, synopsis and cross-referencing between files.
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