Oxalis corniculata, or more commonly Oxalis sp. The worst perennial weed in my garden. I could say it's the only weed in my garden because I have a small garden, and it's easy to weed. Easy to weed, except for Oxalis. Here's a bulb I just plucked from the beets and broccoli.
(Sorry the picture's blurry. I haven't learned to focus manually w/ my camera yet. As soon as I put the ruler in the shot, the auto-focus doesn't know what to do. Here is a shot without the ruler in better focus.)
Fortunately, in my garden Oxalis behaves more like a winter annual than a perennial. It starts coming up now, and by January I'll have it everywhere. And then one day in March or April, it will vanish without a trace. Almost without a trace. According to the California Master Gardener Handbook, oxalis ejects its seeds 10-13 feet! No matter--I will be out there from now until March hopelessly, futilely digging it up and throwing it out knowing full well it will just blow in again from the neighbor's yard next year.
Here's an Oxalis we like. It's the native Redwood Sorrel, Oxalis oregana, growing in this shot with Heuchera maxima and Vaccinium ovatum.
For comparison, each of the leaves on O. oregana are about one inch across.