I heard a while ago that you can use stamps as legal tender to purchase things at the store. This kind of made sense to me--they are government issued pieces of paper with a value printed on them, after all. I did some research, and I think that it's totally bogus now. I found sources that said you could use them, but only if the other guy was OK with it. And you can do that with pretty much anything. I could give you my school notebook in exchange for groceries, and if both parties are happy, then everything's OK.
And what about "forever" stamps. Those could be considered a type of investment, with their value increasing every year.