If you'd rather not read spoilers on whether a cool mod exists, you'd better stop reading this sentence right now because - SPOILERS - it does. If you'd rather not be spoiled as to the name of that mod, you'd probably be better off not reading the rest of this post, really. Because - SPOILERS - it's called the Blue Frog mod. It's for Spelunky, and it turns every character sprite in that game - the player, plants, bats, exploding frogs, the ghost, everything - into the blue frog sprite. That sounds initially dumb, but actually it's clever and great. If you don't want spoilers as to why, you definitely shouldn't read on.
Getting good at Spelunky is about expanding and refining your internal simulation of the game's mechanics. You build a mental wiki of details - 'Cavemen move like this,' 'Bats swoop down like this', 'Tiki traps fire like this' - until you can play out the outcomes of every procedurally generated obstacle. It's what allows you to navigate and improve at navigating a world that's shifting around you with every new life.
The Blue Frog mod masks the appearance of the enemies and obstacles around you, but obscuring the sprites has two effects. The first is that it's totes adorbs. Look at that frog hanging from the ceiling, you'll gasp. And then when you see the ghost, oh wow. Spoilers.
The second is that the mod exposes Spelunky's underlying ruleset. That blue frog hanging from the ceiling? You can work out that it's a bat without being able to see that it's a bat. That blue frog sat still on a single block-wide platform; is it a man-eating plant, or a snake? And can you work that out fast enough not to be killed by it?
SPOILERS: Probably not. The Blue Frog mod makes Spelunky harder than it already is and it's already pretty hard, but like a blind taste-test does for food, the simple graphical change forces you to be more aware of the game's mechanics, and everyone needs to be more aware of the neat, tightly-wound mechanical design of Spelunky's traps and enemies.
I love it. You can download the mod via this Reddit post from the tweak's creator, and the post also contains simple installation instructions.
Are there any other mods which, through the same or different means, have the same effect?