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Doom Legacy
http://legacy.newdoom.com/
I recently found this. It's basically the original DOOM (Or DOOM 2, Final DOOM, Hexen, whatever game u have made with the same game engine), only heavily upgraded.
Resolutions up to 1024x768, in OpenGL, Dynamic lighting, translucent floors etc.
Controls can be fully adjusted, and u can play it like any modern FPS; mouse n keyboard.
Although I haven't tried yet, this should also have a good and still popular online play system. There are loads and loads of custom add-ons, DM maps, and even TCs.
Currently a new version is being written in C++.
HIGHLY recommended for fans of DOOM that would like to get nostalgic feeling, without having to count pixels...