Start indoors and transplant out once minimum soil temperature is reached:
eggplant, leek, onion, pepper and tomato (you should have all of these seeded indoors by now)
Direct seed once minimum soil temperature is reached:
beans, beets, carrot, lettuce, parsnip, pea, radish, spinach and turnip
Variable crops (start 2-6 weeks before soil temperature is reached, or direct seed at optimum soil temperature): cabbage, cauliflower, chard, cucumber, muskmelon, pumpkin, squash and watermelon.
There are exceptions to every rule and a tremendous range of variables that make pushing the envelope both exciting, successful or remarkably devastating. Remember that seed is cheap (relatively) and that time is irreplaceable. Do run the risk of direct seeding peas, do not run the risk of setting out plants in which you have invested months of indoor care and nurturing.
Frankly, it is scary posting such a generalized message. Really, the most important message here is that you should invest in a soil thermometer. This tool is one of Montana gardener's best friends. And, Evan's Ace Hardware reliably stocks this garden tool.
Do not hesitate to send any inquiries regarding specific crops or gardening situations. We will reply as quickly as possible. Do not forget to add 1+ weeks of hardening off to any seedlings that you have started indoors.