If you have opened packets of seeds left over from last year (or even further back) it's useful to know which will still germinate well and which won't. Of course it all depends how they have been stored, but the following list will give you a fairly good idea.
How long will seeds keep ?
Beetroot 4 years
Brassicas 3-5 years
Beans 1 year
Carrots 3 years
Celeriac 6 years
Celery 6 years
Chard 4 years
Cucumbers 7 years
Leeks 3 years
Lettuce 3 years
Onion 4 years
Parsley 1 year
Parsnip must be fresh
Peas 1 year
Pumpkin 7 years
Spinach 1 year
Squash 7 years
Swedes 1 year