John Carmack is ready to release the source code engine Doom 3

doom 3 sources are ready and tested, only lawyers are waiting for their release

So, it will soon be possible to write your games on the id Tech 4 engine, as Carmack promised in 2007.
Despite the fact that the first game on it (Doom 3 itself) was released in 2004, the engine is still considered modern. Prey 2, which is in development, uses it. id Tech 4 supports 5 platforms: Windows, Linux, OS X, PlayStation 3 and XBox 360. The main features of the engine include full pixel-by-pixel lighting, self-shadowing and mega-textures.