I am in zone 6 we have had a string of warm years.

I have had good lock with bronze fennel, lavander,sage, rue and a rosemary thet lived a few years.

Are you looking for culinary herbs, medicinal herbs, or either?

FataMorgana

I would consider either.
Laurie

How about a rosemary plant? You can get an upright or a creeper a/k/a prostrate.

Arp Rosemary: Although rosemary is considered a tender perennial in most plant books, 'Arp' Rosmarinus officinalis has proven winter hardy as far north as Washington, D.C. The plant is a relatively slow grower; heights eventually reach 36" to 48" and widths of 18" to 24", and it likes well-drained soil in a sunny loction.

And best of all, rosemary is a great culinary herb that can be used in many recipes. It is one of my top five herb favorites.

I found these just flipping through a book on herbs - rosemary, thyme, sage, lavender, bay, hen & chicks, bayberry, eucalyptus, agave, artemisia, juniper, holly, citrus, cedar, tea (Camellia sinensis), boxwood, aloe, gardenia, oregon grape, patridge berry, bamboo (sort of evergreen), pine (various), etc.....

Check for hardiness and culture. Go to a library and find a copy of The Herb Society of America New Encyclopedia of Herbs and Their Uses by Deni Brown. You are sure to find more plants that retain their foliage.

FataMorgana

Another nice guide is "The Beginner's Herb Garden", also by the Herb Society of America.

Here is a link that might be useful: The Beginner's Herb Garden

When you say low-growing, how low are you thinking? Many of the herbs do grow up more. (Someone mentioned bronze fennel, which is a beautiful useful and ornamental, but tall and invasive) You could give the creeping rosemary a try--it would be pretty and useful. You're at the low temp end of hardiness, but if you're talking south and full exposure, you may have the right microclimate for it. Thymes are fun--there are so many to pick from. I don't know how they stay during winter for you, as an evergreen. Orange balsam, lemon, variegated--you can go with some different types, or there are all the ornamentals to pick from.

And then there is oregano, makes good ground cover.

You want LOW growing, Not to take over, ...I will scratch rosemary , oregano and lavender out.
My suggestion is either thyme or sage. Sage is a better evergreen than thyme and has a nice silver green color.