This is a Longlegged Sac Spider (Cheiracanthium), possibly C. inclusum, which I found in my back yard under the rim of a flower pot saucer.
Look at those front legs. It is very aptly named.
The longlegged sac spider is one of the more commonly seen spiders in homes. They like to hang out on the walls and ceilings and are most active at night, but they are just as common outdoors in foliage. Rather than build a web to snare prey, they hunt on foot, patrolling around looking for bugs to eat (or for partners to mate with). When they want to rest, molt, mate or lay their eggs, they build a little silken "sleeping bag" around themselves. Although they will bite if handled roughly or pressed against the skin, they will flee if given the chance and are not considered to be dangerously venomous to humans or pets.