My own Quake 2 modification. Has a lot of unique features.

Source code: https://github.com/gildor2/Quake2

For more details, please visit http://www.gildor.org/en/projects/quake2

• Support for game levels (maps) from other games: Quake 1, Half-Life, Quake 3, Kingpin. Those levels are playable with standard Quake 2 game and mods, level entities are converted into Q2 at loading time.
• Transparent support for Quake 3 player models: when one of clients will not have one, or when client is playing with different engine (original Q2 etc) - standard model will be used. Q3 player models has a possibility to use Q2 weapon models.
• Full network compatibility with original Quake 2 servers and clients.
• Improved user interface: it's still looking like Quake 2, but has things originally missing there, like level screenshots etc.
• More customization of player, for example a possibility to stylize railgun effect (like Q3A does).
• Possibility to customize levels, mainly to add FX to it. Called "level patch".
• Some eye-candy features like lens-flares, directional lighting etc.
• Heavily optimized renderer (actually made a entirely new renderer), using fixed pipeline OpenGL functions. One of features is correct dynamic directional lighting for game models.
• Gameplay features like "camper sounds", "falling sounds".
• Reviewed collision and game physics system, fixed many bugs there (and sorry, some exploits used by "hardcore gamers" will not work anymore).
• Lots of debugging console and 3D stuff which is possibly not interesting for most of people.

More detailed list of additions to the engine could be found in 4.XX_Changes.txt.

I didn't find a good name for this engine modifications, don't like things like "Quake 2 Pro", "Quake 2 XP", "Quake 2 2019", "Super Duper Quake 2" etc. So I simply called it "Quake 2 4.0", as the recent vanilla Quake 2 has version 3.21.