Poison Oak
Rhus diversiloba (also Toxicodendron diversiloba)
There is a lot of variety in the precise appearance of this plant. Unfortunately that is problematic because it is also highly toxic to many people and very widespread. The effects range from a small itch to large-scale rashes. Interestingly, some people appear to be immune to the effects of poison oak. The cluster of three oak-like leaves are a hint you may be near it. Also, be aware that oils (the substance which causes the ill-effects) are still toxic even after the leaves have fallen off the plant, such as in the fall. The oils also can transfer from one surface to another to exercise extreme care around poison oak.