Toxicodendron radicans; Anacardiaceae (sumac family)

It may be reviled by humans, but poison ivy is an important plant in our ecosystem: many birds eat the fruit, and some animals will eat stem and leaves, as well.  The flowers aren’t particularly remarkable, but poison ivy is one of the earliest plants to turn color in the autumn.  Watch for blazes of red going up into trees.  If it’s on your property in an out-of-the-way location, leave it be.  Spend your efforts rooting out invasive aliens instead.

Other plants in the sumac family: cashew, mango, and pistachio.  Yup.

Wikipedia has a decent article about urushiol.

Although I haven’t conducted a rigorous scientific experiment on myself (nor am I likely to), in my experience Tecnu is effective should you come in contact with poison ivy.  I always have a bottle on hand.  Use exactly according to instructions.