Last year when we moved in I planted a flower mixture (or rather several from Martha Stewart) and this came up last year. Well it self seeded and I would like to know what this is. :)
It's hard to tell, there's so much "going on" in the picture. The leaves that are predominate in the pic do not belong to white flower in question so you can't go by the leaves. My first thought was a single flowered white stock. Are the flowers fragrant? It's definately in the crucifer family.