Honesty (Lunaria annua) is one of my favourite self-seeders. Once you you have Honesty you will probably never be without it. Years ago I scattered some seeds in the back garden taken from a garden where it has been popping up for decades. This healthy specimen has appeared by the front gate, probably a progeny of one at the back.
 L. annua is biennial and this one put on a particularly vigorous rosette of leaves last year from which grew an unusually thick stem this year which is proving to be very flowery. It attracts the occasional bee but Honesty -being of the Brassica family- is particularly noted as a butterfly plant.