I've got one for you folks: this strange plant is growing in my brother's garden and I assume it is a weed since he didn't plant it - can anyone tell me what it may be?

- Leaves: ovate, long, and opposite. Leaves are all green and look a lot like those of common perennial plants (coneflowers, etc.)

- Growth habit: it has put out a lot of stems from the base, and it is growing in an herbacious manner (not a woody manner.) It also grows fast.

- Flowers: This has to be the give-away. It is currently in bloom, and the flowers are simple, yellow, and hang like shallow bells downwards from the flower stems. The flowers are NOT in large groups or sprays, and each one instead attachs to the main stalk, resulting in the flowers being mixed in among the leaves vs. growing above the plant or at the terminal point of a stalk.

I have no idea what this thing is. I assume it is a weed and should be eliminated, but I've never seen a weed like it, either.