I found this critter on our bathroom wall a few weeks ago.
It's a Yellow Sac Spider, aka Black-footed Spider, of the family Miturgidae. They are very common in US households. Their bites are venomous and cause localized tissue necrosis which basically means an open wound that can take several weeks to months to heal. Yellow Sac Spiders are not aggressive. They typically bite in defense. Their commonality and the tissue necrosis effect is believed to be the cause of many spider bites being blamed on the Brown Recluse. Brown Recluse bites have similiar (and worse) effects, but are much less common.
These spiders don't build a web per se, but a "sac" of silk that they use as a lair. If you've every lifted up an old picture on your wall and found a little web "pouch", it is likely made by this species. I have had encounters with these guys in many places including my car. On several occasions I've been driving along and one crawled across the inside of my windshield out of my defrost vents! Arachnophobia + venomous spider + driving = potential disaster!