looks like possibly a member of the mustard family. unlikely to have been planted in your garden on purpose. I would pull it before it goes to seed as it will produce zillions of seeds if it is a wild mustard such as white or garlic mustard.
Silver Surfer..what do you think?
K
thought of something else:)
if it is a mustard it might have started out as part of a mesclun or salad mix of greens as mustard greens are often included in such mixes. either way, it has bolted to 4 feet and is going to seed so I would get rid of it before it sows itself all over the place.
K