We saw several of these birds today, but this is my best shot. We got off without a bird book, so was unable to identify this bird in the field. When we returned home, I looked it up to see what it was.
Marc was commenting as I was thumbing through the book that it was probably some kind of flycatcher. He was right. This is the Eastern Kingbird.
The bird book calls these birds Tyrant Flycatchers (family Tyrannidae) because of their "bold aggressive behavior..., notably [in] kingbirds."
Besides the coloring, the identifying mark for this bird is the white stripe at the tip of the tail.