This all depends on what area of the world you live in. In the U.S., some dangerous spiders include the Black Widow spider. A bite from a black widow can cause serious illness and even death. There are two types...they both have an hourglass design on the back of their fat, bulbous bodies. The ones with the red hourglass can hurt, but their poison is nowhere near as potent as that of the yellow backed black widow, which can cause death. DON'T quote me on this though...some people say that the color makes a difference, but many more people say that it doesn't matter...BOTH are poisonous. Typically, a bite from either one wouldn't really be deadly unless you were allergic to them, or if you incur more than one solid, venomous bite. Regardless, you should always be aware that Black Widows are dangerous. Black Widows are found all throughout the United States, though the more venoumous ones tend to occur in the Mid to SouthWestern US.
Brown Recluse are not household spiders...they live outside under rocks, leaves and twigs. They are brown, large and hairy with bulbous bodies. Hence the name 'Recluse' they will only attack when their home is disturbed. Brown Recluse ARE dangerous, and their bites can cause death. They are quite prevalent on the east coast of the United States, particularly the Mid-East Coast, from pennsylvania southward to Florida.
Wolf Spiders are vile looking spiders that have elongated bodies and visible fangs. They have long, stick-like legs and can run quite fast. They often make their way inside a house during the summer months. Their bites are not deadly, but can cause a serious sting. They are located throughout many areas of the U.S.
Your common variety Garden Spider is a rather ungainly looking creature...at first glance it appears to be a small tarantula, with a hairy black body. Their bites can cause serious pain and swelling, and if left untreated can cause painful infection. They are often found in grass and gardens and other wooded areas, and make their way into the house during warmer months. They, like many other spiders, have the ability to make their bodies go completely prostrate and flatten themselves to fit into VERY tight spaces...I once had a horrifying encounter with one when I opened up a dictionary that had been sitting unused downstairs for a few years, and the ugly little wretch sprang out.
In the Midwesern and South Western united states, you have tarantulas of various sizes that can cause serious pain and infection with a bite. Desert Tarantulas are particularly bothersome and make their way towards houses whenever they feel like it. They don't usually make their homes inside houses unless you have a cellar. They will usually find a dark, moist area in the cellar to set up home. They'll take care of your mouse problem, but will also be a potential threat to young children and pets. They may also reside in your attic if they can find a way up there. You can easily identify a tarantula: a large, vile looking arachnid covered with hair. They usually grow to be larger than the palm of your hand and can vary in speed. If they are threatened, they will rear up their fore legs to appear larger. Try not to kill them if you find one in your house...just try to get a net over it or capture it in a jar and toss it outside...it'll probably go away. If you decide to squash it, you'll have a heck of a mess on your hands to clean up.
Jumping Spiders (I don't know their class name) are not very dangerous, but can be identified by their thick, bulbous bodies and rather short legs. They move very quickly...and can leap far distances. They very rarely bite, and if they do bite its nothing more than an unpleasant sting. But it is a common spider in the house.
There are many other types of spiders that make their way into the house, and you can find out more by reading up on them on many websites. While I don't know of any off the top of my head, you can run a search of 'household spiders' and come up with quite a few.
Side note: Julia Gregory, PLEASE don't give answers negative ratings simply because you think you know more...i'm not an expert in spiders in any way whatsoever, but when it comes to many venomous spiders, and particularly Brown Recluse, I know what I am talking about and i'm only stating things I know as FACT. If you feel that you can contribute to this question, then do so by submiting your own answer. Don't abuse the rating system by marking my answer wrong to make snooty comments or 'correct' me when i'm not wrong at all. Be in full possession of facts before you mark other users' answers wrong. The website you placed in your comment even supports my original answer.
Comments
The Brown recluse is not a large spider and in some areas of the US they are quite common in homes. http://www.tpwd.state.tx
by Julia Gregory on February 17th, 2004
NIce info. And as for the other person's comment, Brown Recluse can be somewhat large, and are NOT common in homes.
by James Larrick on February 17th, 2004
brown recluse has a fiddle shape on its back and the can get large for a household spider
by TRAVIS SIMPSON on February 19th, 2004
Great detail and info. I never had a spider problem until we built on 5 acres of wooded land-ICK, ICK, ICK!
by Andy Is Wicked Married to Penal Colony on June 12th, 2004
Very well said..............props my friend....:-)
by tOnY mOnTaNa on July 5th, 2004
This is a very useful answer. I don't understand why people give negative ratings to good answers...we're not all experts.
by MikeD81 on July 6th, 2004
!..
by katscratcedme on January 11th, 2005
There are some good ones in Australia.
by mungobaby on January 16th, 2005
At Ft Bragg, I fell into an old hole and came up with 7 BW's on my arms and back. I was in MOPP 4 so not bitten---scary....
by RJTRIES on March 10th, 2006
omg. i would freak out if i had any black widows hangin on me lol
by mac on October 24th, 2009