At first I thought it was Blue Pimpernel, but their leaves are more narrow and pointed, but now it's looking more like Calibrachoa which I thought was sterile.
SOLVED: What is this volunteer from last year?
It very well may be calibrachoa. I had them two years ago, and the year after (and I'm in zone 7), I had several volunteers that looked very much like yours. The baby plants all flowered a magenta color, instead of the colors of the different parent plants.
Looks actually more like hypericum than calibrachoa. Hoping I'm wrong. Still waiting for it to flower.