This is an easy-to-grow bushy annual from tropical South America (ht 30-60 cm), which has a mass of starry blue flowers with a white centre in summer and autumn. It belongs to the Solanaceae family of plants. It flourishes best in sun but will still flower quite well in part-shade; and it grows in most soils. Mulching helps keep the roots cool and moist. It is rarely seen for sale: if you are able to obtain even a single seedling from the garden of someone who grows it, you will have it forever. I started with one such lone plant and now have literally hundreds of them coming up each spring. Otherwise, try to obtain a packet of seeds.
They obligingly fill in any spare spaces in the garden, without swamping surrounding plants. I find that it is possible to dig up even quite large plants and transplant them elsewhere, if a reasonable amount of soil around the root ball is dug up and the plants are watered well for a few days afterwards. Browallia americana mixes effortlessly with any other colour but is especially fetching with yellow, orange or cerise flowers. When I pull them out at the end of autumn, I shake the plants around to ensure another crop the following spring. Some plants can even bloom into winter in Sydney.