Forthysia is the first that comes to mind, but I'd think of that as spring rather than summer. Potentilla isn't bell-shaped, nor is buddleia or erigonum.

Can you tell us more, such as, was it in full sun or shade? in the wild or a park or a roadside or a garden? Was it on a hillside, or near water? Where did you see it, I mean which part of which state? -- maybe we can narrow the choices by using climate as a guide, was it in mountain, valley, plains, hill country?

Sounds like forsythia to me too. Ours is gorgeous right now. It's planted along side a barn/shed in our back yard. The yellow really pops next to the barn red color. Beautiful.

Thanks meldy and lemon. I will definitely check out forsythia. The shrub I saw was in a garden in front of someones house, in full sunlight, in Ontario, Canada. It was mid-summer. I liked it so much, I just have to have one.

hi! doesn't sound like a forsythia to me. not a shrub, but how about lysimachia?