Looks exactly like a fresh marijuana sproutling. I can't see what other plant would be growing for you. How it got there? *shrug*
It does look like a marijuana plant, just without serrated leaves. It's definitely not a tomato plants and neither any other vegetable I've seen, but I haven't seen that many.
You could take it out of that pot and keep growing it just to see what the hell is that. Or you could ask in a gardening forum, maybe somebody else can identify it.
haha i know im going to once i transplant my other plant outside. I think it would be fuckin awesome if it was some weed genetically altered by thc or some shit haha cause it is sooooo crystally.