Shortly before we participated in the Extra Life campaign, Carter told me about one of the heavier mods to the classic Doom games, called Brutal Doom. Strictly speaking, modding these games is nothing new -- there have been fan levels, replaced graphics, and total conversions out almost as long as the game itself. However, that really went up a notch in more recent years, when the game's programming was released Open Source.
Brutal Doom needs GZDoom to run, which by itself is basically an emulator that allows the older games to be run on 64-bit computers, as well as adding controls for mouse aiming, jumping, and crouching. BD takes it a few steps further, by updating the weapons and monsters to the point where just about every kill is extremely bloody and gruesome -- and yet kinda cool to watch, as well.
If you have the wads for Doom and Doom II, all you need to do is copy them into the same directory as these emulators. The level themselves are exactly the same as before, in their claustrophobic and hellish glory. But now the renewed controls make them feel even more timeless -- even if the addition of jumping over obstacles or being able to shoot at vertical angles does render some of the puzzles a bit superfluous (particularly in the final level of Doom II). This is a renovation that gives an old classic a breath of fresh air, while not taking away from what made it a classic in the first place, and is an absolute recommend.