Spreading fleabane has a floppy, prostrate habit that makes it a good choice for the front of a garden bed or as filler inbetween taller plants. It is smothered in pale purple disc-shaped flowers starting in late June to July, and the show will continue as long as there is some moisture in the soil. It is biennial; that is, it is short-lived and completes its life cycle in two years. However it will reseed (read below).
 
This species thrives son disturbance, and is a must-have if you are working with a large empty area that is otherwise going to fill up with weeds. Spreading fleabane can fill those areas and help keep the weed seedlings from taking over. That being said, it is a superb naturalizer, and the seeds are wind-borne. You will find little spreading fleabane popping up in random places, but in my opinion that is its greatest strength. It does need bare ground to reseed, so as your plantings mature this species might not be as prevalent. Easy solution is to collect some seeds and sprinkle them where cover is needed.
 
There are too many good companions to list, but start with curlycup gumweed, hairy golden aster, blanket flower, and yarrow.