Got some old, unplanted seeds lying around in envelopes from last year, or even the year before?  Wondering whether they’re viable?  University of Minnesota Extension Service to the rescue!  Here are some general guidelines from my “Vegetables” textbook:

Seeds that last 5 years:
Broccoli
Brussels sprouts
Cabbage
Celery
Chinese cabbage
Collard
Corn
Cress, water
Cucumber
Eggplant
Endive
Kale
Kohlrabi
Watermelon
Lettuce
Muskmelon
Radish
Rutabaga
Spinach
Squash
Turnip

Seeds that last 4 years:
Beet
Mustard
Pepper
Pumpkin
Tomato

Seeds that last 3 years:
Asparagus
Bean
Carrot
Pea

Seeds that last 2 years:
Parsley
Okra

Seeds that last 1 year:
Leek
Onion
Parsnip
Sweet corn

I should put a giant * here, and say that these averages are based on storing your seeds in nice conditions — that would be room temperature, with low humidity.  If they smell rotten or funny they are probably no good.  If you kept them in your garage, the temperature swings probably did them in.

I would also still sow seeds just a little more thickly if they’re older.  You can always thin out the seedlings later.  But this should save you some money if you have some old seeds lying around that you can use up.