I just started out fiddling with other people's mods in 2005, but kept fiddling and adding for the next 7 years; there's nothing left of their work at this point.
Currently I have Trent Reznor's soundpack (with shakes and volumes corrected, and adding to vanilla monster sounds rather than replacing them), decals and shell casings stay (including delayed colour fade for pentagram and explosion marks), bodies stay (except Cacodemons which look terrible when left sitting about, or monsters that respawn or are gibbed by explosions/doors), modified bullet holes, glowing plasmagun/BFG, short-range chainsaw illumination, modified pickups so each provides a very small amount but you can carry everything you find [trying to heighten the survival-horror aspect], stopped Z-sec zombies jerking your vision all over the place, increased the health and rebalanced the damage of virtually everything, rebalanced every weapon so none of them go obsolete, lowered late-game reliance on the Soul Cube, fixed a few bugs, fiddled with giant monster sounds to make them more frightening, added PerfectedDoom3's heightmaps, and even had a crack at creating new sounds for the Archvile (they don't work very well so I'll repeal them next time I play, but they are certainly f**ked up).
Doom 3 becomes a very different game when the enemies are all strong enough to take a pounding, there's a lot more skill involved, and it greatly heightens the game's inherent apprehension. I'm sure I don't need to tell you that, though.
