Looks like a sedum. Try the Name That Plant forum for more specifics.

If it is a mound that does not spread it looks like Sedum kamtschaticum.

It has spread all over in a wonderful way. That's why I posted under groundcovers. I'll try another forum. Thanks.

But has it spread steadily outward or has it seeded all over in the wonderful way that S. k. does, forming new plants.

It was explained to me when I visited that a few plants were planted around this water feature and those plants have grown each year and expanded. Some have now run into one another and covered certain areas.

I have those. I am pretty sure it is sedum. It does expand easily without much effort.

Hello. I started some cuttings a neighbor gave me that looks exactly like that. In late spring they are covered in small yellow flowers. He told me its called Sedum Golden Carpet.