I have these all over my yard - front and back. We had them last year (our first with this yard) as well. The distinctive features are the reddish stems and and they're sticky. When you pull them up they always stick to your gloves.

They don't have a scent and tend to prefer cracks in the steps and walls - damp shady spots.

More pictures at: http://www.flickr.com/photos/sharonpriest/sets/72157630292064446/with/7444568048/