Hi,
Any seed can be frozen. It must be properly dry before freezing. Purchased seed will be dry enough since it has been in storage for awhile. Seeds from the garden take a little time.
Smaller seeds will probably be properly dry in a week or two. Larger seeds though can be deceiving. Like beans can be dry, but the moisture content may not be low enough for freezing. To check for readiness, taking a few out side, putting them on a hard surface like a driveway, and hitting them with a hammer. If they smush, they aren't dry enough. If they shatter, they are dry enough to freeze.
Once in the freezer, they should be kept in a moisture proof airtight container. When taking them out of the freezer, you must let the container come to room temperature before opening. If you don't do this, you will kill the seed. It is also not a good idea to be taking the in and out a lot because seeds don't like lots of temperature swings.
So this method is good for very long term storage, and when you are only going to take the seeds out once a year for planting. If you are planning on trading seeds, this is not a good method.
Remy

Question Remy...not all can be frozen, can they? Tropicals shouldn't be, should they? I just responded to another thread saying that not all, can, or should be frozen. Should I go eat my words?

Sue

Sue,
Except for tropicals that should stay moist and be fresh sown like citrus, they can be frozen. If you think about it, tomatoes and peppers are tropical plants, many annuals that self-seed over the winter are actually tender perennials : )
So my statement above really should be amended because northern spring emphemerals need to to stay moist though they can be frozen(Though I just had success wintersowing with seeds of Dicentra exima(Fernleaf Bleeding Heart) that someone sent me that were in dry storage.) and some tropicals need to stay moist and should not be frozen.
Though I'm not sure why someone would be freezing tropical ornamentals. Most people freezing seeds are doing it for edibles.
Remy