Quake
Nohealth Mod
Think you kick ass at single-player quake? Try this mod out. It's my crowning achievement in Quake C. I don't know if I have the source anymore, but there isn't much to this one, really.
It makes health packs not work.
This means that all damage you take is permanent. A grunt pops you: you carry that damage forever.
The challenge: start a single-player game on EASY difficulty. Don't bother with anything harder. Try to complete an episode. You won't believe how hard it is.
I did not alter the game mechanic that puts your health up to 50% at the start of each level if you're below 50% at the end of the previous level. Even with that, you won't be able to complete an episode on easy difficulty.
You'll need the nohealth mod. Unzip it and put it in your quake directory, parallel to the id1 folder and your other mods. Start the game by dropping the nohealth folder on the Quake icon, or do the equivalent of "quake +game nohealth" on your OS. And say your prayers.