heh. I was gonna say 'looks like something in the mustard family'. Broccoli seedlings. Sorry I'm not much help.
The suggestion that it was a mustard was correct, these look very similar to cabbage seedlings for instance. Cabbage, kale, cauliflower etc. are all derived from mustards. Pick a leaf and taste or smell it, there will probably be at least a little characteristic pungency discernible.
