Ever visited the Brain Slug planet without a hat? You’re lucky you didn’t (or you might feel lucky you did :P) because, indeed, there are mind controlling parasites around and they are not a nice bunch. In fact, most these parasites lead their hosts to their ultimate demise as unwilling assistants for them to breed or hatch. In most cases, the process involves forcing the host do its best to get eaten by releasing the appropriate chemicals in its brain.
For example, when Toxoplasma gondii infects a rat, it makes it lose its revulsion against cat pee: instead of safely running away from their natural predators, rats feel sexually attracted (for a brief time, too) to cats, so they can be conveniently eaten and, hence, T. gondi moves into a better lodging. And it’s only too good they don’t care about humans, because if you are a cat person, you may well carry these yourself!
In some cases, there’s no need for a change of hosts: worm Spinochordodes tellinii, for example, lives in water as an adult, but their larvae prefer to grow on the insides of grasshoppers and crickets, that acquire them when they drink infected water. When the growing cycle is over and the worms want to return to the watery element, the hosts feel compelled to jump themselves to their watery deaths in the nearest pond, where the parasite can be released and start a new cycle.
Control may even continue after the parasites are no longer inside the body of the host. For example, Glyptapanteles wasps usually infect baby caterpillars with their eggs. When the eggs hatch, they don’t kill the host, but pupate nearby. Rather than destroying the things in an Alien fashion, the caterpillar feels the need to protect them, by enveloping them in silk and keeping dangers away.
These pesky invaders may even change the animal appearance to achieve their goals. The Leucochloridium paradoxum changes the look of one of its snail host head tentacles to resemble a caterpillar, which their next preferred hosts, birds, find just delicious. They also change the snail sleeping habits to bring them out in plain daylight, where birds can spot them easily.
While Futurama has the coolest brain slugs ever, the idea is far from new: Heilen already presented it in his novel The Puppet Masters. It is remarkable that not all brain controlling parasites are bad in scifi: in Aldiss’ Super-Toys Last All Summer Long, parasites kept people from over-eating to avoid obesity. For some reason, I’m betting people would be willing to ingest those, should they exist.
You can get information and (creepy) pics of up to 10 of these species at io9.