Oenothera species. I can't tell you which.

" Oenothera" or Evening Primrose. There are about 145 cultivars of this species and it can be an aggressive plant which some consider invasive.

http://www.wildflower.org/plants/result.php?id_plant=oesp2

kimmq is kimmsr

Thanks a lot, that's very helpful! I guess you wouldn't usually plant this on purpose, it must just be some wild thing that is taking advantage of some space where I pulled up some other very invasive ground cover and haven't really replanted with anything other than a few annuals that don't do very well... But they are not unsightly, I guess I'll leave them be for now and remove them once I have an idea what to do with the area.

Looks like sun drops to me. I have them in my garden and they spread rapidly.