I'm torn between thinking it's a hydrangea and a snowball viburnum. They look very similar. I'm sorta leaning toward the hydrangea after looking at images of both. The leaves and the growth pattern look more like the hydrangea IMO. But I'm certainly no expert. My wife generally knows a lot about this stuff and she didn't think it was a hydrangea at first because it isn't round but then we saw some images of that flower that weren't round either. And the leaves definitely look more hydrangea than viburnum.
Times like this I miss my grandmother who majored in horticulture in college. She knew every plant we could find.
I have found some information that indicates these are hydrangea. Apparently a lot of people have a problem telling them apart though. There are numerous pages detailing the differences and how to tell which is which. Mainly the size of the flower group is the giveaway. Viburnum doesn't get as large as hydrangea in that department. So I'm going with hydrangea with the caveat that I'm really not sure at all. I'll leave it to the experts.