General Products #4 (although I suppose #1-#3 deserve honorable mentions). I’d hate to see how much space a mole of #4 would take up, though. 
eunoia, I recently (in this thread) contended that eludium is the source of many of the peculiarities of cartoon physics. It’s not just for blowing up planets anymore! 
Oddly enough there really is such a thing as Ice-nine.
Phase diagram of water
Spider-Man’s web fluid.
Neotraxin
An all purpose wonder drug that cured just about everything from The Six Million Dollar Man, in the alien Bigfoot episode.
If Neotraxin was such a marvelous wonderdrug, how come Sandy Duncan had to keep taking dose after dose of the stuff just to stay alive?
Cheers, Optihut!! make one for me, too. Did someone mention Eludium Fozdex, the shaving cream atom?
Space Mead, HPL’s creation, allows you able to travel through space without air. Of course you will need to find a byakhee…
Another chemical is in The King in Yellow, in the story The Mask. Turns any living thing into stone when immersed into it, while a ray of sunlight suddenly shines upon it. The vines and the heart of the creature gives off a different shine. After three (or four?) years the creature returns back to its original state.
Whatever was in Dr. Jeckyll’s potion.
Mary Poppins’s multi-flavor cough syrup.
Everlasting Gobstoppers!
This maybe stretching things a bit, but a person on a christian message board said that scientists should come up with vaccine against Homosexuality, and the poster dubbed it “homocillin”
Fake Matter - (Anvil Of Stars) Greg Bear
Heavy Water - The Original Flash (Jay Garrick)
Sci-fi worldbuilding at
http://www.orionsarm.com/main.html
What was the drug called in Harley Davidson and the Marlboro Man, Blue Ice?
“You put it in your eyes and it tells you lies”. Imagine hitting that while sipping moloko plus synthemesc with a little melange?
Waaaaaaaait a minute … heavy water isn’t fictional!
This heavy water was fictional-
it made you go fast-
er …
or is that just a fictional quality assigned to a mysterious but real compound?
oh well, you are probably right
Sci-fi worldbuilding at
http://www.orionsarm.com/main.html
In that case the New Accelerator
(by H.G.Wells)
is properly fictional and does the same thing.
Sci-fi worldbuilding at
http://www.orionsarm.com/main.html
It’s not exactly fictional, having real components, but…
“Remember how we always thought there was no way to kill a toon? Well, the judge found a way. Turpentine, acetone, benzene. He calls it the Dip.”
I’ll second the vote for the Pangalactic Gargleblaster.
What was the name of that drug from ‘Brain Candy’?
mmm…Gleemonex.
No prob…put it inside an Alien and call it molecular acid. Simply lead him to whatever you want to clean/dissolve and make a little pinhole to let out as much as you need.
P.S. Did anyone ever find out what an Alien’s skin is made of???
Actually it’s “Xenovirus Takis A”. It was called the “Wild Card” virus by humans because of the effects, but Tsienne (Dr. Tachyon) who, along with some other members of the Ilkazam house, invented it referred to it as “Xenovirus Takis A” throughout the series. On Takis it was commonly referred to as the “Enhancer” project.
I was going to say Ice-9, too, but of course, it is later than I think. Which reminds me-- nobody’s mentioned “Oxychloride X” from Arch Obler’s inimitable radio program Lights Out. A universal solvent.