Okay, I know it's only January and I'm already obsessing about gardening. But seriously, it's only a few more weeks before I can get started again, at least with seedlings in the house. And if I'm really on the ball I'll be able to get some plants in the ground soon too.
John Jeavons includes a chart in How to Grow More Vegetables that shows the optimal soil temperatures for germination of various vegetables. The earliest, most cold-hardy plants will germinate at soil temperatures as low as 35 degrees F. They certainly prefer somewhat warmer temperatures, but if your goal is to get things started as soon as possible it's good to know which plants don't mind the cold. You're taking a chance, obviously, by planting ultra early, but if it doesn't work out you simply plant more seed a bit later and if it does work out--terrific!
Here are some minimums for you (remember this is soil temperature, not air temperature):
35 degrees F: lettuce, onion, parsnip, and spinach seeds
40 degrees F: beet, cabbage, carrot, cauliflower, celery, chard, parsley, pea, radish, and turnip seeds
50 degrees F: asparagus, corn, and tomato seeds
60 degrees F: bean, cucumber, eggplant, okra, pepper, squash, and melon seeds
Where I live (in Colorado) soil temperatures are hovering somewhere around 32 degrees F. right now. I don't yet own a soil thermometer but luckily the local water conservancy district posts soil temperatures daily. The closest testing site to me is about 4 or 5 miles away, so it's only a rough estimate but good enough until I get my own thermometer. If you check with your area cooperative extension they may have this sort of data for your area or may be able to point you to an organization which does. Our water conservancy district compiles a lot of really useful data--soil and air temperatures, wind, humidity, solar radiation, precipitation, and more (if only I knew how to interpret all of that!).
Last year our soil temperatures started consistently hitting 35 degrees in mid-February, and 40 degrees in mid-March. I've never tried planting that early but I might experiment this year just to see what happens. I haven't decided if I will take the lazy approach, just planting some seeds and forgetting about them (except to water) or if I'll be more involved and offer them some protection, keeping an eye out for hard freezes, covering and uncovering and otherwise coddling them.
I've never been good about getting anything planted early, but it would be so nice to have those early greens and peas and radishes this year.