Dog Violets (Viola riviniana) pop up all over the place in sun or shade, town or country. Like Red Deadnettle mentioned a few days ago they seem to be especially prolific this year. It might be my imagination but I think there is a greater variation in colour than usual- many hues of blue/purple/mauve. There is also a pure white form.

 The unscented V. riviniana was probably called 'Dog' Violet to imply it is inferior to the fragrant Sweet Violet (Viola odorata). That's a bit harsh!