From Wikipedia (not a political topic, so only take with a few cups of salt):
The Brazilian wandering spider (Phoneutria spp.) is an aggressive and highly venomous spider regarded by some as the most dangerous spider in the world. Phoneutria spp. is actually a genus with five known similar species whose members are highly venomous and not reluctant to attack people who appear threatening.
The effects are reported to range from only a couple of painful pinpricks to a full-blown envenomation. In either case, people bitten by a Phoneutria or any Ctenid should seek immediate emergency treatment as the venom is possibly life threatening. The Phoneutria fera and nigriventer are the two most commonly implicated as the most virulent of the Phoneutria spiders. The Phoneutria not only has a potent neurotoxin, but is reported to have one of the most excruciatingly painful envenomations of all spiders due to its high concentration of serotonin.
This spider is called the wandering spider because it wanders the jungle floor, rather than residing in a lair or maintaining a web. This attribute is another reason it is considered so dangerous. In densely populated areas, Phoneutria species usually search for cover and dark places to hide during daytime, and this may occur inside houses, clothes, cars, boots, boxes and log piles; thus generating accidents when near people.