Common Teasel, (Fuller's Teasel) Dipsacus fullonum is a good possibility. See photos and information, http://oregonstate.edu/dept/nursery-weeds/weedspeciespage/teasel/common_teasel_Dipsacus_sylvestris_page.html
There are other plants with similar looking seedlings. Unless you planted them, they are probably a weed and you should remove them before they get any larger. Pull a couple now to see the root structure. A deep or wide-spreading root signals difficulty removing self-seeding weeds. Early is always best.
It is interesting to find out for sure what you have. If you want to do that, I suggest you pot one up, or have one isolated to watch it for a few more weeks. The next few sets of leaves should make identification more of a certainty. Post updated photos here if you need more help.