It's Oxalis strica, Yellow Wood Sorrel. Very plentiful here on the prairies.

Thank you, Pudge. It does look like a member of the oxalis family. It did not appear in my garden until about the last 5 years or so - maybe imported from the prairies!! We have been having a lot of rain here and after every downpour I have to be pulling them out of my veggie gardens. It is quite persistent.
Northerner.

I just pulled some of this yesterday for the first time (I think). I thought it came out too easy to be clover.

Is this a plant I should encourage?

Since getting an ID on this plant, I have found it described as a common Ontario weed. I have read somewhere that it is edible (I don't know how), but I would not encourage it in my garden. It produces many, many, seeds as any good weed does, and can be quite difficult to remove because it loves to grow in shade.

I have lots of this "weed" as well. It is pretty with small yellow flowers, but a rampant seeder, so I am constantly pulling it up, mostly from my tiny lawn. It grows all over my property, though, in both sun and shade.