Mutagenic Goo

"This was back in the day when science goo could just do anything! You could make up the most absurd reasoning, that has no scientific logic, but, it's science goo! So it flies."

—The Nostalgia Critic on TMNT's ooze.

This is any liquid/slime/sludge form of Applied Phlebotinum that causes living creatures that touch it to somehow mutate or transform into... something else. Occasionally, it may need to be ingested (or injected) to take effect, but in most cases it can be easily absorbed through the victim's skin—their own Lego Genetics will take care of the rest.

The goo comes in many forms—it may be toxic nuclear waste, some form of alien bodily fluid, mud from a cursed swamp, or contaminated water carrying The Virus.

Common results include Body Horror, Painful Transformations, and Lovecraftian Superpowers. However, it's also a fairly common backstory for non-Squicky superheroes.

Differs from Super Serum in that the effects of this are usually unexpected and nearly always unwanted.

Examples of Mutagenic Goo include:

Film

The "bio-fuel" from District 9 that turns Wikus into a prawn.

Live Action TV

The slime produced by the Wirrn in the Doctor Who serial "The Ark in Space", which turns humans into Wirrn.
The titular heroine of The Secret World of Alex Mack gains her powers when she is accidentally splashed with toxic waste.

Tabletop Games

Magic: The Gathering has the Phyrexian oil that kickstarted the Phyrexian invasion of Mirrodin.

Toys

Energized Protodermis from Bionicle is either this or an Acid Pool, depending on whether you are destined to transform or not. Also, the water in the Pit would slowly mutate beings into Fish People, although the effect on organic creatures would be much slower and less noticeable.

Video Games

One form of Phazon in the Metroid Prime series is like this. Other times it appears as crystals. The mutations it causes appear to be due to long-term exposure to a type of radiation it emits.
The Potion of Mutation in Dungeon Crawl. Might be a good mutation, might be bad.
The content of canisters and the Geneforge pool was most often blue, other times green, ochre or magenta, but always gooey and a catalyst for mutation.

Web Original

The Nostalgia Critic also provides an example in the same review that the page quote comes from when he combines what he calls science goo and a jar of Philadelphia cream cheese; and it turns into a 12" talking Dennis Miller doll.
Handwavium, the mystery substance underpinning Fenspace, can have this effect exactly once per person. Because what comes out the other end is somewhat unpredictable, it's usually used this way as a last-ditch, everything-else-has-failed medical treatment.

Western Animation

The Ooze that transformed the Teenage Mutant Ninja Turtles from regular turtles into the awesome ninjas we all know. A common threat in the show was Retro-Mutagen Ooze, which Shredder and other villains tried to use to turn the turtles back into their regular non-mutated selves.
Futurama has a mutagenic lake in the sewers of New New York. It turns rats into flying fish-pigs, and a purple octopus living there claimed to have once been a little blonde girl named Virginia. Sewer mutants are, of course, immune, and so use the lake as a punishment against offensive outsiders.