Lamb's Lettuce - we have plenty in our garden this time of the year, gets weeded out and the leaves nipped off and put into my sandwiches. The blue flowers are TINY, and the stem /leaf structure is very distinctive: two opposite leaves, from the middle of which branch out two stems. If you want to be nerdy, it seems like a fractal structure, as this continued division repeats all the way along the stem.
Geoff