The quintessential reseeding annual of northern climes is the poppy. Both
papaver rhoeas (shirley poppy, corn poppy), and papaver somniferum (opium
poppy) will reseed themselves for years in cool climates. If your climate is
cold enough, iceland poppy may reseed itself too. (Native to arctic
regions). Bachelor buttons, alyssum, california poppies, godetia, cosmos,
lavatera, and chinese forget me nots will all reseed themselves, although
not always in the spot you would have chosen for them. The trick is learning
to recognize the seedlings when young, and also not to be afraid to hoe
often, since these plants will reseed themselves too generously and it's
nice to give them a little breathing space. (The ones the hoe misses will be
plenty!).