white vinca minor. http://farm3.static.flickr.com/2577/4144665169_0acdf816bb.jpg it's fairly invasive, but easy to remove (shallow rooted)

I have Thyme, grows between the flagstone..but the grass keeps growing in it and I am always pulling out the grass..hope you do a better job getting rid of the grass. I love the thyme, at the moment I am trying to find out about now to transplant more of it as some of it has died. Good luck with yours..it is very beautiful.martha/zucchini

I second the Ajuga suggestion. I have Chocolate Chip and Caitlin's Giant planted in an area that gets bright shade with no direct sun, and in another area that receives the full effect of our HOT Southern sun in the Summer. It does just great in both spots. Pennyroyal is another good spreader and it smells nice when you step on it. Gotta watch it though as it can get out of hand and spread quickly to places you don't want it to go. My brother planted a tiny bit outside his backdoor and the next thing ya know it had claimed a sizable chunk of his backyard. But you can mow over the stuff just like grass. (ie., it takes heavy foot traffic and abuse just fine.)

I've heard good things about hardy plumbago/ dwarf plumbago (Ceratostigma plumbaginoides.) However, I don't have it, so can't speak from experience. It has attractive blue flowers, and the foliage turns color in fall. But it supposedly dies off in the winter, to re-emerge late next spring. Here is a link that might be useful: hardy plumbago