This is an annual called Torenia. It must have re-seeded.
I saw some in a planter in the "bird nest" structure in Beijing last night on the olympics. they were blue.

Agree Torenia. It is an almost care-free annual that self sows readily.

I thought it was an annual......it is coming up everywhere now........which is great.......except I planted some coneflowers .....their spot actually moved by about 2 feet from where they were planted last year.........but it is a nice surprise.........glad I didnt pull them up thinking it was a weed.........thanks for the plant name!!