This is a female long-legged sac spider (Cheiracanthium mildei) I found in my bathroom a couple of days ago. She was dead when I found her, and the abdomen has a flattened, deflated appearance, which happens after death. This one's body was 7 mm long (not counting the legs).
These spiders arrived in the U.S. from Europe in the 1940s. They are very active at night and will emerge from their daytime resting sacs to run along walls and ceilings in pursuit of prey (insects).
Other common names are yellow sac spider and black-footed spider. They have been erroneously associated with necrotic bites and should no longer be considered a species of medical concern.