Some Sedum, some Hypericum, Delosperma, prostrate forms of Rosmarianus. There are other drought tolerant groundcovers, but are not "aggressive" at all, spreading fairly slowly. Even some of the above species are not super aggressive, but they do spread reasonably fast (in my garden, anyway).

If you really want something aggressive, try Mexican Evening Primrose, Oenothera speciosa. I saw a picture of an area bordered by concrete and the plants were gorgeous and they choked out everything.

I don't know about aggressive, but ornamental thyme is fairly fast growing with the advantage of Spring/ Summer flowering, depending on the species. It looks good all year and tolerates your zone in Cottonwood quite well. Go to www.highcountrygardens.com to see their use of ornamental thyme as a ground cover. It is fabulous.