Tesseract

Tesseract is a fork of the Cube 2: Sauerbraten engine that remains open-source and significantly improves on the graphics and fluidity of the game. This version introduces a good deal of new features which make Tesseract an excellent choice for those who wish to experiment with FPS builds and more.

Simply put, it’s an excellent game engine

Right off the bat, Tesseract looks great, and there’s no denying that. Jumping into the editor mode you’ll get to enjoy a broad range of high-quality objects and textures that are very easy to manipulate, add, remove, reshape and whatever you need to do.
It’s all relatively straightforward if you’ve come across other game engines. And even if you’re just starting off with this one, after you put in a couple of hours, it becomes a breeze to work with. To list a few of the things that it offers, the most notable are screen-space ambient occlusion, deferred MSAA, HDR rendering, real-time diffuse global illumination and Oculus Rift Support.
The game engine does require considerably more hardware resources to function as it should, after all, any changes you make, structural, lighting, etc., are rendered in real-time. Bottom line, if you want to put some effort into it, Tesseract can help you assemble something very good.

As a first-person shooter, it lets you enjoy fast-paced combat and fluid gameplay

Playing Tesseract is just as great as it is playing with it. Probably its biggest setback is that the servers are almost dead, barely populated but that shouldn't stop you from creating a local game with your buddies or strangers and get shooting. If that’s not an option, there are bots you can add into the game, to which the difficulty level is adjustable, and they will put up a good fight if you want them to.
Tesseract runs very smooth, and if you’re into fast-paced shooters, this one will do the trick. It’s no UT or Quake, but it certainly comes close. There are a handful of levels and two weapons to blast with, but then again, it’s open-source, no one is stopping you from introducing new models, texture or any kind of content you need.

A wonderful learning environment

To wrap it up, Tesseract is one of the best things you can find when it comes to game engines. It’s easy to work with, it provides quality material and switching into game mode can offer you a good deal of fun. It’s a very well balanced game/game-game engine.