S-8472 is Voyager's biggest example of biowank. They have more than 100 times the DNA of humans (which in real life doesn't prove anything, amoebas have over 100 times as much genetic material as humans). They're telepathic. Their "bioelectric fields" wrecked havoc with Voyager's equipment. A few stray cells slapped onto Harry Kim grew to become many tenticles covering his face and upper body, slowly eating him alive. They were able to easily resist Borg attempts to assimilate them through their aggressive immune system destroying the Borg nanoprobes. One member of their species managed to mangle a large pile of drones by slashing them with its claws. And they can survive in the vacuum of space for an undetermined period of time.
S8472 are so powerful that Voyager's crew estimated that they would completely destroy the Borg collective in a matter of months. Despite their power, however, they are surprisingly timid. Voyager's holo-doc modified some Borg nanoprobes to be undetectable by 8472 tissues in order to treat Harry, and the modified nanoprobes also proved an effective weapon against the bioships. S8472 forces subsequently retreated back to their realm after taking less than two dozen casualties--I guess with all their awesome abilities, they only wanted to fight when their opponents really couldn't fight back. Cowards.
S8472 have appeared in Trek canon on at least two subsequent occasions. One of them was left behind after the big retreat. The Hirogen, a hunter race, decided the thing would make good prey, and it was rescued by Voyager. The rest of the species decided the Federation was a threat because of the modified nanoprobes, and 8472 planned on infiltrating Earth. Voyager stumbled across one of their simulation outposts in the Delta Quadrant and talked them out of their plan, convincing them that diplomacy between S8472 and the Federation could be a wiser course of action.