Let me be the first to share my ideas here. One of my personal favourites for filling gaps is definitely the Alyssum - I've seen them in different colours from white and green to purple and even red, including many stunning variations of these. That means you can choose whatever fits your garden landscape best. Also see Moss rose and Purslane - they look amazing for me and you might like them too.
Other beautiful annual ground covers I myself like: Lobelia, Verbena, Periwinkle, Nolana, Cineraria. Most of these are fairly popular among gardeners and landscapers, so I guess they won't be difficult to find. More options would be Petunias and Tropaeolum - some call it Nasturtium but it's really the same plant anyway. Not sure if these two are suitable for your area though - Snoopea and Poached Egg flower.
If it's a rather shady area that you need covered, then I recommend you to try Hardy begonia or Corydalis if you're looking for more interesting colours. Oh, and if some of the flowers mentioned don't sound familiar, just google them - that would be a great way to see how different varieties and colours look, too. Hope this helps and good luck!