By the looks of the leaves I would say it was Sedum spathulifolium possibly Cape Blanca, but there are others like it, differning only in the colour of the leaves.
Palustris, I think you might be correct. If you look real close at Poppy's photo you can see the whorled petals of a Stonecrop of some sort down underneath the blooms. There is also a Sedum spathulifolium (Pacific Sedum) that it could be. Sedum acre (Biting Stonecrop, Wallpepper) is a lookalike. I find info on dozens of Sedum and a great majority of the Sedum are a variety of Stonecrop. And most of the flowers look pretty much exactly alike with the difference being yellow as opposed to white and other colors, and the shape of the water-storing leaves.
The leaves you can see in the picture are 'greyish'. Those of sedum acre and its different forms are green with occasionally white markings. Most of the other Amereican Sedum with that kind of flower have reddish green leaves. The only one it can be is Sedum spathulifolium. There are different forms of this where the flowers are the same, but the leaves are more or less purple from S.s.'carneum to S.s.'purpureum'. I collect Sedum, by the way.
I'd say Palustris got it right the first time. We've got three different Sedum spathulifolium, don't know the varieties.
