Yep, the silver certificates were supposed to be exchangeable on demand for hard money. They are moderately collectable depending on condition -- it's always worth hanging on to one. A friend got one in change from a casino a couple of years ago.
I think the feds destroy them as they come through.
Also, half-dollars still had silver in them up until about 1970 or 1971, even though the rest of the coinage had been turned into copper/nickel by 1965. Worth saving them; most people don't notice, and they'll fetch a little more than the new ones.