Cube
This is an oldy-but-goodie. Cube was first released in singleplayer mode in January 2002 and had multiple updates all the way up to 2005. The game and its engine (which also called Cube) were developed by Wouter van Oortmerssen, a Dutch computer programmer. The game was always very well reviewed and it even received an award for "Best Free 3D Action Game" in 2003. It runs on almost all operating systems, it features both singleplayer and multiplayer gameplaya and even has a build-in level editor.