I found this growing in my raised garden, where I had tomatoes, basil and geraniums last year. It's approximately 2'X2' right now and growing. It resembles parsley.
Click on photo for full size
Volunteer plant that resembles flat-leaf parsley - and has a sibling, although a brighter green!
Friends, after researching your suggestions/answers, and taking some more additional pix, I've come to the conclusion that this is most likely poison hemlock (http://www.ars.usda.gov/services/docs.htm?docid=9975). As you can see from the additional pix I snapped today, it has smooth hollow stalks with purple blotches and no hairs on it's stems and now has many flower heads on the flower spike. How it arrived in my organic garden, one can only guess...perhaps through a bird dropping? In any case, it will be eradicated this weekend (precariously!) and destroyed (http://homeguides.sfgate.com/kill-hemlocks-30900.html).
It's brighter green sibling does smell like chrysanthemums when crushing the leaves, so perhaps it is feverfew, but due to the close proximity to the poison hemlock, it will likely get pulled, as well. Question: can I now safely plant/consume my organic vegetables in this space, after having poison hemlock in the raised garden?
Thank-you again for all your educated responses! I shudder to think what would've happened had we consumed this 'parsley'! 
Something in the Fumaria family. I have it too, and your post made me finally look it up. So thanks.
Do the leaves for the one on the right smell like crysanthemums when they are crushed? If so, this is probably Feverfew - Crysanthemum parthenium . When it blooms, if you haven't already removed it, post another picture and we can confirm. Feverfew has groups of small white flowers. It is benign (except it can spread where it is "happy") often perennial, and some folks use it for medicinal purposes.
The plant on the left is definitely an umbellifera - that is, it is a plant in the carrot and parsley family. You can tell by the leaves, and the way they join to the stem, by clasping around it and forming a sort of swollen node. It looks to be close to blooming, too - the main stem's internodes are beginning to lengthen into a blooming stalk. The problem here is, members of this family can be very difficult to tell from one another, even for experienced gardeners and outdoors folk, but I agree this is NOT parsley. My concern is it may be hemlock, which is poisonous, and also an invasive weed. Here is a picture and an identification page for poison hemlock. Have a look and see if you agree it looks similar. If you do agree, I'd definitely get rid of it now, before it blooms, and dispose of it in a way that pets, livestock and kids can't get to it.
Giant Hogweed has much broader leaves, so I don't believe that is the correct ID. If you ever do find any, btw, you may need to call the county to come pull it for you, as contact with any part of the plant's juices causes SEVERE dermatitis and, if you accidentally rub any in your eyes, blindness.
After seeing the purple blotches on the stems I can also confirm poison hemlock. That's a cool, but pretty wicked plant! One of the most deadly in North America. People don't often confuse it with parsley, but sometimes confuse it with queen Ann's lace. Not a good mistake to make either way. It can be deadly in pretty small quantities.
Remove the plant. Be cautious about not eating small parley like seedlings and you'll be fine. I actually had a garden once where poison hemlock was a pretty common weed visitor. The chemicals will not transmit through the soil to your next plants so no need to worry.
