Alyssum, also known as sweet alyssum, Lobularia maritima, has really grown on me. They aren't native, and they do readily reseed, but I haven't heard of them being invasive around here. These are all volunteers growing where the parent plants dropped seeds last year, and they are some of the last blossoms in the yard.
Looking closely at them this year, I realized: oh, these are in the mustard family!" See how as the flowers open upwards, they leave a long stem of seed-pods? And also each flower has four petals.
All mustards are edible, but after one taste I have to report that I won't be eating alyssum salads any time soon.