General (none specialized on FPS like Doom, Quake, ...)
- Simple DirectMedia Layer (SDL) provides cross-plattform (even mobile) access to audio, video, input devices, written in C and works in C++, Python, C#.
- Simple and Fast Multimedia Library (SFML) has very similar features to SDL but may look a bit more modern.
- libgdx seems to be very popular currently and is written in Java and C/C++. It's a whole application framework and runs in many environments (mobile and desktop) with, so they say, very good performance.
- Cocos-2dx written in C++, seems to be quite popular, also among professionals.
- Allegro is a cross plattform audio and video framework and they stress that they aren't a game engine.
- Ogre3D C++
- LWJGL Java
- MonoGame in C# and based on XNA?
- Qt is definitely not a game engine but can be used as one. We use it currently.
- Unity is not open source but immensely popular and quite powerful and kind of low budget (they make money via the asset store or services).
- orx 2D game engine written in C/C++.
- Godot engine is a game maker written in C/C++.
- Polycode C++/Lua game engine.
- LÖVE is a 2D game engine in Lua.
- Urho3D 2D/3D game engine written in C++.
- PyGame is based on SDL but not very popular and some say it's slow.
- Blender game engine Blender also has a game engine obviously.
- OpenFL projects writen in Haxe (don't know it).
- HaxeFlixel not sure what it is actually.
- Flixel
- Enigma based on Game Maker.
- jMonkeyEngine written in Java and geared towards 3D.
- Panda 3D strongly geared towards 3D and using Python.
- Arianne a multiplayer online game framework.
- SDL based game engine. Inactive since 2014?
- Dash Engine in D.
- Maratis3D C++, Lua, 3D graphics centric
- Octaforge C++, Lua, 3D graphics centric
- Enigma
- Superpowers
- Marauroa
- Delta Engine C#
- Crystal Space 3D domain defunct wikipedia
- Quad Engine 2D graphics C++, C#
- Irrlicht C++
- Lumix C++
- FS2 open engine Freespace 2
- Box2D Java
- Z Game Editor
- CR Editor for Ogre3D