I was recently sent the Sarah Ravens cutting garden brochure, and she helpfully lists some annuals that she sows now for a show lasting from mid to late summer until the frosts.
So I have shamelessly copied some of her suggestions (but either used seeds I already had, or got some cheapies on eBay), and have sown some more short multistemmed sunflowers, snapdragons, zinnias and some art shades calendulas. I've also sown some biennials: chocolate hollyhocks and wallflowers (from Barnsdale seed swop thank you!).
There were lots more suggestions, it might be worth sending off for a copy! ;D