Surprise Flowers on an Already Pretty Plant

I have this plant in two windows boxes. One box gets a lot of sun, the other is mostly in the shade, and they're both growing like weeds. So much so that I've had to prune them back a few times this summer. They are both giving me these tiny purple flowers! I bought the plant because the foilage is so pretty ... I had no idea they would flower. What a nice surprise it was to find these tiny blooms!

I can't remember what kind of plant they are, but I really like them! As tall as they get, they really aren't suited for window boxes, so I probably won't plant them there next year, but aren't they pretty?!!