I feel like I should probably know what this is already; it looks somehow both really familiar (could we have sold these at the garden center?) and really foreign (surely I'd remember seeing these before, if they were as weedy as they appear to be?). These have been coming up around town in the last couple weeks, especially in ditches and vacant lots.
Not an uninteresting plant, in any case. If you need a closer (or better-focused) look, the picture is much bigger when opened in a separate window.
UPDATE: Okay, duh. I am informed in comments that this is Queen Anne's lace, Daucus carota. Which makes perfect sense. I guess I've never known what it was unless it had flowers on it, and haven't paid the foliage any attention before.