Spiraea x vanhouttei
The identification is suspect as this is a cutting from an un-named parent and there are many forms and hybrids of white Spiraea but it fits the description in the various expert publications I've consulted, so I will go along with that. Please comment if you think differently.
Regardless of that, this is a shrub that well deserves it's place in our gardens, distinctly unfussy once established it gives a magnificent floral display at this time of year and remains an attractive foliage plant thereafter. Mine has thrived with little attention other than a little pruning, keeping it tidy and removing some older wood later in the year. Propagation is easy from side shoots after flowering and I've read that some can be propagated by root division but I have never put this to the test. The rain outdoors is currently making every effort to batter this one to the ground but I'm confident it will recover quickly afterwards.