A couple years ago we had some flagstone installed for paths in our front yard. That fall we planted groundcover, each smaller than the size of a quarter and about a foot apart. We weren't sure how well they'd do, but it was a relatively small investment.
Now it's about a year and a half later and the groundcover is thriving. The ones with flowers are blue star creeper, and the dark green one in the foreground is wooly thyme. The blue star creeper grows very aggressively, working its way into the chips we use as mulch. You can also see it working its way into the wooly thyme.
The wooly thyme is no slouch, however. It's planted on the path that goes off to the right, and is doing a good job of starting to cover the flagstone. We also planted some in small spaces between the flagstone, and those are starting to thrive.
We also planted blue star creeper between the flagstone; on that path there's an inch gap between all of the stones. Those didn't do as well, but the creeper coming in from the sides of the path will fill those areas in quickly, anyway.
This picture was taken about a week ago, and already the blue star creeper is now completely covered in flowers. It's a very nice look, especially contrasting with the darker green of the wooly thyme around the other path.
The instructions with the plants said that they can handle light traffic. Interestingly enough, the tags said they can be stepped on up to three times per day. Fortunately, we're not so concerned to the point of counting how many times each plant gets trod on!