People that do not spend a lot of time in the outdoors often see the long legged house spider, also called the cellar spider. The cellar spider is often found in damp locations like basements, crawl spaces and cellars, which is how it got its common name. Cellar spiders are tan or gray in color. Like all spiders, they have eight legs.
When food supplies in their environment are insufficient, these spiders travel to other webs and pretend to be trapped insects. As the other spider attempts to catch and consume it, the cellar spider attacks the unsuspecting arachnid. Also known as vibrating spiders, cellar spiders utilize wobbly, vibrating movements to confuse predators and attackers.[7]
Like most spiders the cellar spider produces silk for webs and venom to help catch its prey. The cellar spider is small and has tiny fangs but they can bite and puncture our skin, so this debunks part of the myth where they can't bite us.
Now to the point that their venom is the most poisonous and can harm people. To date there have been no verified bites that have caused any harm to people, and from studies with mice their seems to be no lasting affects besides slight site irritation. Below is a video of the show MythBusters testing both the poison and ability to bite that is stated in the myth.
http://www.discovery.com/tv-shows/mythbusters/videos/daddy-longlegs-minimyth/