From the folks at Botanical Interests:
In early fall, it pays to keep an eye on nighttime temperatures, so you aren't caught off guard and can get the last of your crops harvested in time. Here's a simple list of common vegetables and their frost tolerance.
Light Frost – Temperatures 28-33 degrees F.
Hard Frost - Temperatures below 28 degrees F.
May be damaged by light frost: Beans, Cucumbers, Eggplants, Muskmelon, New Zealand Spinach, Okra, Peppers, Pumpkins, Summer Squash, Sweet Corn, Tomatoes, Watermelon
Can withstand light frost: Artichokes, Beets, Calendula, Carrots, Cauliflower, Celery, Chinese Cabbage, Endive, Lettuce, Pansies, Parsnips, Peas, Snapdragons, Sweet Peas, Sweet Alyssum, Swiss Chard
Can withstand hard frost: Broccoli, Brussels Sprouts, Cabbage, Collards, Kale, Kohlrabi, Mustard, Onions, Parsley, Peas, Radishes, Spinach, Turnips
Plants can be protected from a few light frosts with row covers or blankets.
Mulched beets, carrots, leeks, radishes, and parsnips can be harvested later in fall before the ground freezes. Frost makes leafy greens and root vegetables sweeter, so it's worth leaving some of your kale and carrots in the ground until your ready to use them.
Early fall is a great time to sow salad crops in a cold frame for harvest in late fall and early winter. Bok choy, lettuce, mesclun, kale, mustard, and spinach are a few good varieties to try.