Nigella, or 'Love-in-a-Mist' to give it it's common name, is very easy to grow, provides excellent ground cover, and will give a good display of blooms throughout the summer, if you keep dead heading, rather than let them all go to seed. You can also use the dried seed heads in winter displays.
If you do let a few heads go to seed, you'll be able to save them, and save yourself some money the year after.
Nigella - Love-in-the-Mist