Looks a lot like a radish seedling to me.
However, if I'm not mistaken, it doesn't even have its first true leaves yet, making identification very tenuous.
It can also be any number of brassica species, e.g. mustard (l) or cabbage (r):
If you can't wait to see what it grows into, smash one of the "leaves" between your thumb and a finger. If it's a radish, it will smell like one. If it does, taste the other leaf. If it tastes radishy, you have your answer without waiting. If you want to wait, however, doing that would kill the seedling.
Brassica seedlings will also taste spicy, but not like a radish.
Based on the number of "weeds" in the brassica family, I'd guess it belongs in that group.
I included these diverse possibilities to illustrate how difficult identifying a plant without true leaves is. :)