This is one that I've only begun to notice. The crdamine hirsuta or the Hairy Bittercress. It is distinctive, poking up in fields and sidewalk cracks alike with its happy white flower and longer erect spikes.
It is flowering now, but in the winter has a basal leaves that are round growing before the snows melt. It's apparently edible, but has to be cooked twice. Or something like that. I'm still investigating.