Three of the most lethal mushrooms belong to the genus Amanita: the death cap (A. phalloides) and destroying angels (A. virosa, and A. bisporiga); the fool's mushroom (A. verna) and two are from the genus Cortinarius: the deadly webcap (C. rubellus), and the fool's webcap (C. orellanus). Several species of Galerina, Lepiota, and Conocybe also contain lethal amounts of amatoxins. Deadly species are listed in the List of deadly fungi.
The following species may cause great discomfort, sometimes requiring hospitalization, but are not considered deadly.
Amanita muscaria (fly agaric)?known for its use as an entheogen, and it is now known that the toxins convert to psychoactives upon drying  .
Amanita muscaria var. regalis?symptoms generally mild. 
Amanita pantherina (panther mushroom)?contains similar toxins as A. muscaria, but is associated with more fatalities than A. muscaria  .
Chlorophyllum molybdites (greengills)?causes intense gastrointestinal upset.
Entoloma (pinkgills)?some species are highly poisonous, such as livid entoloma
(Entoloma sinuatum), Entoloma rhodopolium, and Entoloma nidorosum. Symptoms of intense gastrointestinal upset appear after 20 minutes to 4 hours, caused by an unidentified gastrointestinal irritant. 
Many Inocybe species such as Inocybe fastigiata and Inocybe geophylla contain muscarine, while Inocybe erubescens is the only one known to have caused death.
Some white Clitocybe species, including C. rivulosa and C. dealbata?contain muscarine  .
Tricholoma pardinum, Tricholoma tigrinum (Tiger Tricholoma )?gastrointestinal upset due to an unidentified toxin, begins in 15 minutes to 2 hours and lasts 4 to 6 days.
Tricholoma equestre Man-on-horseback?until recently thought edible and good, can lead to rhabdomyolysis after repeated consumption.
Hypholoma fasciculare/Naematoloma fasciculare (Sulfur tuft)?usually causes gastrointestinal upset  but the toxins fasciculol E and F could lead to paralysis and death 
Paxillus involutus (Brown roll-rim)?once thought edible, but now found to destroy red blood cells with regular or long-term consumption  .
Boletus satanas (Devil's bolete), Boletus luridus, Boletus legaliae, Boletus piperatus,
Boletus erythropus, Boletus pulcherrimus?gastrointestinal irritation. Of these, only B. pulcherrimus has been implicated in a death. Many books list B. erythropus as edible, but Arora  lists it as "to be avoided."
Hebeloma crustuliniforme (known as Poison pie or Fairy cakes)?causes gastrointestinal symptoms such as nausea and vomiting.
Russula emetica (the Sickener)?as its name implies, causes rapid vomiting. Other Russulas with a peppery taste (Russula silvicola, Russula mairei) will likely do the same  .
Agaricus hondensis, Agaricus californicus, Agaricus praeclaresquamosus, Agaricus xanthodermus?cause vomiting and diarrhea in most people, although some people seem to be immune. 
Lactarius piperatus, Lactarius torminosus, Lactarius rufus?these and other peppery-tasting Lactarius are pickled and eaten in Scandinavia, but are indigestible or poisonous unless correctly prepared. 
Lactarius vinaceorufescens, Lactarius uvidus?reportedly poisonous. Arora  reports that all yellow- or purple-staining Lactarius are "best avoided."
Ramaria gelatinosa?causes indigestion in many people, although some seem immune. 
Gomphus floccosus (the scaly chanterelle)?causes gastric upset in many people, although some eat it without problems. G. floccosus is related to the chanterelle and sometimes confused with it. 
Keep in mind, this is in no way meant to be taken as a complete list of posionous mushrooms! These are the most common ones but new species of mushroom are still being discovered. There are also various other kinds of fungi that can be harmful and possibly deadly. If you wish to learn more about ne of these species, simply google it^.^ Peace!