So @naseef asked this in another discussion about engines available to us on Linux. So I thought I would make a big list of libraries, frameworks, engines and tools. I have no DOUBT that I will have missed many great tools. So please comment below so I can extend this list even further! Keep in mind that it MUST run on Linux. It however does not have to be free.
Programmer Text Editors/IDE's
Gedit - Open Source
Kate - Open Source
Atom - Open Source
Geany - Open Source
Visual Studio Code - Open Source
Micro - Open Source; Terminal Text Editor
Vim - Open Source; Terminal Text Editor(There is a GUI version)
Nano - Open Source; Terminal Text Editor
GNU Emacs - Open Source; Terminal Text Editor
Code::Blocks - Open Source
QtCreator - Freeware
KDevelop - Open Source
JetBrains - Open Source/Freeware/Paid; IDE's for various languages C/C++, Java, C#, Go, Python and more
Eclipse - Open Source; Primarily used for Java but can be used for other languages as well with the appropriate plugin
Netbeans - Open Source; Primarily used for Java but can be used for other languages as well with the appropriate plugin
SDL2 - Open Source; Bindings for various languages available
SFML - Open Source; Bindings for various languages available
GLFW - Open Source; Bindings for various languages available
Allegro - Open Source; Bindings for various languages available
LWJGL - Java; Allows you to use lot's of libraries with Java for OpenGL/OpenGL ES/Vulkan/OpenAL etc..
OpenAL - Open Source
FMOD - Paid; For sound/audio unsure if the editor works on Linux. However the library does.
WWISE - Paid; For sound/audio unsure if the editor works on Linux. However the library does.
AppGameKit - Paid; Raspberry Pi version is free
BlitzMax - Open Source; Requires symlinking to 32bit libs
BlitzMax-NG - Open Source; Updated version of BlitzMax. Must compile yourself
Love2D - Lua
PyGame - Python; Currently being upgraded to SDL2
Raylib - C
Tile Engine - Variety of languages(C/C++,C#,Python)
Ultraviolet Framework - C#
MonoGame - C#
Polyplex Engine - Open Source; Uses the D language. Made by our very own @Clipsey
FNA - C#; Drop in replacement for Xna
LibGDX - Java
Irrlicht - Open Source
Ogre3D - Open Source
Horde3D - Open Source
Panda3D - Open Source
Starling Framework - Open Source
Urho3D - Open Source
Phaser 3 - Open Source; WebGL Game library
Gosu - Open Source; Ruby/C++
BabylonJS - Open Source; Web Game Library
MelonJS - Open Source; Web Game Library
Armory3D - Uses blender very nice little engine. Has a few ways to code both visual and with HaXe
JMonkey Engine - Java
Godot Engine - GDScript/C#/Others
Ren'Py Visual Novel Engine - Open Source
Flare Engine - Open Source
Atomic Engine - Open Source
Flame - Open Source; cRPG Engine
Senlin - Open Source; cRPG Engine
Candle - Open Source
BYOND - Open Source; Client for Linux is in beta
Cube1/Cube2 - Open Source
DimensionX - Open Source; MMORPG Engine
OpenXray - Open Source; Implementation of the Stalker Engine
OpenRW - Open Source; Remake of GTA III engine
HPL1 Engine - Open Source; By Frictional Games, powers the Penumbra series.
Play Canvas - Open Source / Can pay for hosting; Web Engine
Stratagus - Open Source; RTS Engine
Torque2D - Open Source
Torque3D - Open Source
OpenRA Engine - Open Source; Make old 90s style Westwood games up to Tiberian Sun
SpringRTS - Open Source; TA inspired 3D RTS Engine
OpenRTS - Open Source; RTS Engine
Lumix Engine - Open Source
Banshee Engine - Open Source
Closed source / Commercial Engines
Defold - Lua; Free
Esenthel - C++ Like scripting; Paid monthly/yearly subscription can get source code
Unity3D - C#; Monthly/Yearly subscription
Unreal Engine 4 - Blueprints and/or C++; Royalty payment and you can get source code.
V-Play - Free/Paid Subscription
Tombstone Engine - Paid
Elflight Engine - Paid
Stencyl - Free/Paid Subscription
Unigine - Paid
Tiled - Fantastic open source map editor.
Blender3D - Open source; Fantastic for modeling! Can be used for compositing, animation and more.
Modo - Paid
3D Texture Painters
Armory Paint - Open Source; Powered by the Armory3D engine. Paint your models in real time. See changes with every stroke.
Mari - Paid
Gimp - Open Source; Fantastic image editor
Krita - Open Source; With paid versions on non free platforms/stores
Inkscape - Open Source; Best Vector editing tool on Linux
MyPaint - Open Source
Synfig - Open Source
Dragon Bones - Open Source; Like Spine but open source
AnimeEffects - Open Source
Spriter - Free/Paid
Aseprite - Open source/$15; Must purchase precompiled version or compile it yourself.
Slate - Open source
LoSpec Pixel Editor - Freeware web based pixel editor. Can use predefined palettes on LoSpec.
Grafx2 - Open Source; Runs on most OS's including Haiku! Started on DOS. Very powerful if you can get used to the interface.
Piskel - Open source; Runs on the web but can be run locally.
Pixel.Tools - Open Source; Collaberative pixel art editor that runs in your web browser.
Swanky Paint - $10.99
Audacity - Open Source
KWave - Open Source
Bitwig - Paid
LMMS - Open Source
Ardour - Open Source
Klystrack - Open Source
DefleMask Tracker - Open Source
Adlib Tracker II - Open Source
SunVox - Freeware/Paid
LilyPond - Open Source
Rosegarden - Open Source; Great scoring tool, needs to be plugged into a synth program for playback
Yoshimi - Open Source; Fairly sophisticated synths
Kdenlive - Open Source; Powerful video editor; TLG approved
OpenShot - Open Source; Simple, easy to use video editor
Shotcut - Open Source
Flowblade - Open Source
Lightworks - Free/Paid
Natron - Open Source; Powerful open source compositor
Nuke Studio - Paid; Widely used by the film industry.
DaVinci Resolve - Free/Paid
Boom Library - Paid
Font Squirrel - Free/Paid; Be sure to check the license before using!
The League of Moveable Type - Open Source; All free but sadly a limited collection
Fonts by Mark Simonson - Paid
The Ultimate Oldschool PC Fonts - Free; Check the license at the bottom of the page.
MeshLab - Open Source
MakeHuman - Open Source
TexturePacker - Free/Paid; Used to pack your sprites into optimized tile sheets.
SpriteIlluminator - Free/Paid; Normal map editor designed for 2D games.
Open Game Art - Collection of art/sound assets for non commercial/commercial use.
Thanks to everyone below who contributed
@nanodragon9999, @ukbeast89, @Chrozayis, @Clipsey