I've lost the label for this plant, but I think it is a variety of sundrops, Oenothera, perhaps the species is fruticosa. The flowers are nice and bright and open all day long. The foliage is slightly variegated or mottled with gold.
Or, who knows, this could be something else completely. Like I said, I've lost the tag.
I do know this self sows a bit, but with some hoeing up in the spring, I can contain it to more or less this one clump, so I wouldn't be afraid to share a start of it with others.
If someone recognizes this as something else, please comment!