These little annuals (ht 10cm), which look like miniature pansies, are great favourites of mine. Common names include heartsease and Johnny-jump-ups. I originally grew them in my cottage garden years, but they still come up every year, as they self-seed readily. The flower of the basic species has a jaunty yellow whiskered face with deep purple 'ears'. There are often varying shades of lavender on the faces and all the colours combine to make a pretty effect. The plants can be echoed by nearby flowers which bloom at the same time, such as purple flag Iris or yellow daisies.
They like reasonable soil and will flower best in full sun. They bloom from late winter until late spring. I have found that it is easy to transplant them to other places in the garden when the seedlings are small. A lovely cultivar is known as 'Bowles' Black', which has velvet near-black flowers with a golden eye. It often comes true from seed.