This year I'm growing three different wildflower mixes; midwest,
northeast, and western. Three of my boxes were planted last fall. I've
grown the midwest mix for many years but not that familiar with the
northeast and western mix. Anyway, a plant has popped up in areas seeded
with western mix that I think I remember as being a common weed here in
Chicago Zone 5 but I'm not sure. In one area there's a clump of these
plants and I'm usually careful about pulling things I'm unsure of.

Here's a pic of the very young plant.

http://brandylion.com/images/weed-leaves.jpg

The underside of the leaves are green with purplish veins.

I don't necessarily need the exact name of this (but if you know it that
would be nice), rather, is this a common weed or not or is this some sort
of plant that grows out west and I should be happy it's growing?

BTW: I've googled and googled for weed ID resources and haven't been able
to find any good sites. Google is becoming infected with ads in their
searches. Are there any good plant ID sites where you can check off a
list of characteristics and the site tries to match that with possible
plants? I remember using something like this last year but I seem to
have forgotten to bookmark it.